The most detailed collection on the ruling of the Shia by past and present scholars can be found here :
http://www.sunniforum.com/forum/showthr ... d-Twelvers
Interestingly "a sufi" forum. I will copy some of the statements compiled in that topic here for the lazy ones amongst us
Verdict of Imam al-Shaf'i
سمعت البويطي يقول: سألت الشافعي: أصلي خلف الرافضي ؟ قال: لا تصل خلف الرافضي، ولا القدري، ولا المرجئ.
قلت: صفهم لنا قال: من قال: الايمان قول، فهو مرجئ، ومن قال: إن أبا بكر وعمر ليسا بإمامين، فهو رافضي، ومن جعل المشيئة إلى نفسه، فهو قدري
"Yusuf ibn Yahya al-Buwayti said, 'I asked al-Shaf'i (may Allah have mercy on him), 'Can I pray behind a Rafidhi?' He said, 'Do not pray behind the Rafidhi, nor the Qadari, nor the Murjiyi'. I said, 'describe them to me.' He said, `The one who says that Iman is statement [only], then he is Murjiyi`. And whosoever says that Abu Bakr and 'Umar (may Allah be pleased with them) are not the two imams, then he is a Rafidhi. And whosoever places the Will for himself, he is Qadari."
(Siyar A`lam al-Nubala', 10:31)
Verdict of Imam Ahmad bin Hanbal
780 - وأخبرني عبدالملك بن عبدالحميد قال سمعت أبا عبدالله قال من شتم أخاف عليه الكفر مثل الروافض ثم قال من شتم أصحاب النبي لا نأمن أن يكون قد مرق عن الدين
"Imam al-Khilaal said: 'Abd al-Malik ibn 'Abd al-Hamid informed me saying: I heard Abu 'Abd Allah [Imam Ahmad] say: 'Whosoever reviles the Companions, then I fear disbelief for him like the Rawafid'. Then he said, 'Whosoever reviles the Companions of the Prophet, then we do not believe he is safe from having rejected the Religion'.”
(Sunnah lil-Khilaal, 3:493)
أخبرنا أبو بكر المروذي قال سألت أبا عبدالله عن من يشتم أبا بكر وعمر وعائشة قال ما رآه على الإسلام قال وسمعت أبا عبدالله يقول قال مالك الذي يشتم أصحاب النبي ليس لهم سهم أو قال نصيب في الإسلام - 779
"Abu Bakr mentions I asked Abu 'Abd Allah concerning the individual that verbally abuses Abu Bakr, Umar and A'ishah. He replied 'I do not regard them to be Muslim.' He further says 'I have heard Malik say, 'Whoever verbally abuses the Sahabah of Rasul Allah he has no share in Islam'.”
(Sunnah lil-Khilaal, 3:493)
781 - أخبرنا زكريا بن يحيى قال ثنا أبو طالب أنه قال لأبي عبدالله الرجل يشتم عثمان فأخبروني أن رجلا تكلم فيه فقال هذه زندقة
"Zakariyya bin Yahya mentions he has heard Abu Talib mention that he mentioned to Abu 'Abd Allah concerning an individual who verbally abuses Uthman and people have mentioned this fact to me that he speaks ill of Uthman. He replied they [he ones who speak ill] are zindiqs.”
(Sunnah lil-Khilaal, 3:493)
أخبرني يوسف بن موسى موسى أن أبا عبدالله سئل وأخبرني علي بن عبدالصمد قال سألت أحمد بن حنبل عن جار لنا رافضي يسلم علي أرد عليه قال لا - 783
"Yusuf bin Musa mentions that Abu 'Abd Allah was asked - this was mentioned to me by Ali bin 'Abd al-Samad - He replied I asked Imam Ahmad bin Hanbal concerning a Rafidi neighbor who greats me , should I answer his Salam? He replied 'No'."
(Sunnah lil-Khilaal, 3:493)
786 - أخبرني محمد بن الحسين أن الفضل بن زياد حدثهم أن أبا عبدالله قال الرافضة لا تكلمهم
"Muhammad bin al-Hussain mentions Fadhal bin Ziyad narrates that Abu 'Abd Allah said, 'Don’t speak to Rawafid'.”
(Sunnah lil-Khilaal, 3:494)
Verdict of Imam Malik
Hafiz ibn Hajar Makki (may Allah have mercy on him) writes in Al-Sawaiq al-Muhriqah, p.294,
ومن هذه الآية أخذ الإمام مالك في رواية عنه بكفر الروافض الذين يبغضون الصحابة قال لأن الصحابة يغيظونهم ومن غاظه الصحابة فهو كافر.
وهو مأخذ حسن يشهد له ظاهر الآية ومن ثم وافقه الشافعي رضي الله تعالى عنهما في قوله بكفرهم ووافقه أيضا جماعه من الأئمة
"And from this verse, Imam Malik (may Allah have mercy on him), as reported from him, had deduced the verdict of kufr of Rawafid as they show hatred towards the Sahabah. He further states because the Sahabah causes them anger and every individual who is angered by the Sahabah is a kafir. This extraction is excellent as the verse in itself bears testimony to it. Therefore Imam Shafi (may Allah have mercy on him) has agreed with him (Imam Malik) on the verdict of declaring them [Rawafid] kafir and a large group of Imams agreed with him too."
Scan:
Imam Qurtubi al-Maliki (d: 671H / may Allah have mercy on him) comments after producing the fatwa of Imam Malik,
لقد أحسن مالك في مقالته وأصاب في تأويله. فمن نقص واحداً منهم أو طعن عليه في روايته فقد ردّ على الله رَبِّ العالمين، وأبطل شرائع المسلمين
"Indeed, Malik did well in his statement and he reached the correct explanation. So whoever belittles a single one of them or reviles him in his narration, then he has rejected Allah, the Lord of the worlds and he has nullified the Shari'ah of the Muslims.” (Tafsir Qurtubi, 16:297)
This fatwa of Imam Malik was also quoted by:
1. Imam Al-Khilaal
2. Hafiz ibn Kathir
3. Imam Sayyid Alusi
4. Hafiz ibn Taymiyyah
5. Qadi 'Iyad Maliki
6. 'Allamah Abu Nu'aym
and others (may Allah have mercy on them).
Verdict of Imam Abu Hanifah and Imam Muhammad
Hafiz ibn Hajar Makki (may Allah have mercy on him) writes in Al-Sawaiq al-Muhriqah, p.362-363,
ومر أن أئمة الحنفية كفروا من أنكر خلافة أبي بكر وعمر رضي الله عنهما والمسألة في الغاية وغيرها من كتبهم كما مر، وفي الأصل لمحمد بن الحسن رحمه الله والظاهر أﻧﻬم أخذوا ذلك عن إمامهم أبي حنيفة رضي الله عنه وهو أعلم بالروافض لأنه كوفي والكوفة منبع الرفض؛ والروافض طوائف منهم من يجب تكفيره، ومنهم من لا يجب تكفيره فاذا قال أبو حنيفة بتكفير من ينكر إمامة الصديق رضي الله عنه فتكفير لاعنه عنده أولى أي إ ّ لا أن يفرق إذ الظاهر أن سبب تكفير منكر إمامته مخالفته للإجماع بناء على أن جاحد الحكم اﻟﻤﺠمع عليه كافر وهو المشهور عند الأصوليين وإمامته رضي الله عنه مجمع عليها من حين بايعه عمر ولا يمنع من ذلك تأخير بيعة بعض الصحابة فان الذين تأخرت بيعتهم لم يكونوا مخالفين في صحة إمامته ولهذا كانوا يأخذون عطاءه ويتحاكمون إليه فالبيعة شيء والإجماع شيء ولا يلزم من أحدهما الآخر ولا من عدم أحدهما عدم الآخر فافهم ذلك فانه قد يغلط فيه. (فان قلت) شرط الكفر بإنكار اﻟﻤﺠمع عليه أن يعلم من الدين بالضرورة (قلت) وخلافة الصديق كذلك لأن بيعة الصحابة له ثبتت بالتواتر المنتهى إلى حد الضرورة فصارت كاﻟﻤﺠمع عليه المعلوم بالضرورة وهذا لا شك فيه ولم يكن أحد من الروافض في أيام الصديق رضي الله عنه ولا في أيام عمر وعثمان وإنما حدثوا بعده
"It was already mentioned that the Hanafi scholars condemned one with kufr who denies the caliphate of Sayyiduna Abu Bakr and Sayyiduna 'Umar (may Allah be pleased with them). This ruling is mentioned in their books with detail as it is mentioned in Al-Asl by Imam Muhammad bin al-Hassan al-Shaybani (may Allah have mercy upon him). It is obvious that they have inherited it from their Imam Abu Hanifah (may Allah be pleased with him) and he knows more about the Rawafid as he is from Kufa and Kufa was the origin and headquarter of the Rawafid. Among the Rawafid, there are many groups, some must be condemned with kufr while some not. So, when Imam Abu Hanifah regards the denier of caliphate of Sayyiduna Abu Bakr (may Allah be pleased with him) as kafir, so one who curses him will necessarily be called kafir except that if one makes some difference. As it is clear that the reason of declaring him as kafir is his opposition to the ijma' (consensus) based upon the ruling that one who denies a unanimous matter (of religion) will be called kafir. This is a general rule among the theologians. The caliphate of Sayyiduna Abu Bakr (may Allah be pleased with him) is a unanimous matter from the time when Sayyiduna Umar came forward for his ba'yah (solemn pledge of obedience), but it cannot be contradicted with the delay made by some Sahabah; since those who delayed in ba'yah they did not delay due to any disagreement about his eligibility of caliphate, therefore they used to take his bestowals and used to take their issues to him. So, ba'yah is something and ijma' is something else, and one is not necessary for the other. You should understand this point, as some people commit mistake therein. If you object that calling anyone with kafir is conditioned with the rejection of a matter categorized as ‘necessary in religion’. I will say that the matter of his caliphate falls in the same category; since it is proved from widely reported traditions to the extent of ‘being necessary’ that the Sahabah took oath of allegiance (bayah) with him, so this matter turned like a unanimous matter known ‘necessarily’. And there is no doubt in the matter and there was no Rafidhi in the period of Sayyiduna Abu Bakr al-Siddiq, neither in the reign of Sayyiduna 'Umar nor Sayyiduna 'Uthman (may Allah be pleased with them) rather they emerged later on."
Verdict of 'Allamah Abu Sa'ud
Following is the fatwa of Shaykh al-Islam of Ottoman Empire, Allamah Abu Sa'ud, as quoted by 'Allamah ibn 'Abidin in his Rasa'il.
(may Allah have mercy on them)
أفتى العلامة أبو السعود لما سئل عن الشيعة أيحل قتالهم وهل يكون المقتول منا شهيدا مع أنهم يدعون أن رئيسهم من آل النبي صلى الله عليه وسلم وكيف يجوز قتالهم وهم يقولون لا أله إلا الله: أن قتالهم جهاد أكبر والمقتول منا في المعركة شهيد وأنهم باغون في الخروج عن طاعة الإمام وكافرون من وجوه كثيرة، وإنهم خارجون عن الثلاث وسبعين فرقة من الفرق الإسلامية لأنهم اخترعوا كفرا وضلالا مركبا من أهواء الفرق المذكورة وإن كفرهم لا يستمر على وتيرة واحدة بل يتزايد شيئا فشيئا...فلذا أجمع علماء الأعصار على إباحة فتلهم وإن من شك في كفرهم كان كافرا فعند الإمام الأعظم وسفيان الثوري والاوزاعي أنهم إذا تابوا ورجعوا عن كفرهم إلى الإسلام نجوا من القتل ويرجى العفو كسائر الكفار إذا تابوا، أما عند مالك والشافعي وأحمد بن حنبل وليث بن سعد وسائر العلماء العظام فلا تقبل توبتهم ولا يعتبر إسلامهم ويقتلون حدا
.
"Someone asked 'Allamah Abu Sa'ud about the Shia whether it was permissible to fight with them and will our people killed be regarded as martyrs as they claim that their leader is from the progeny of Rasul Allah and how is it permissible when they recite the kalima 'la ilaha ilal lahu'. He answered by giving the following fatwa, 'fighting against them is regarded as Jihad Akbar and those who are killed in battle against them are declared Shuhada (martyrs). They are renegades against the Imam and are kafir because of many reasons. They are not included in the seventy three groups [of Islam]. They have openly innovated many acts of kufr and falsehood in following their whims and fancies. The decision of their kufr is not based on one issue but daily it increases. Therefore the 'ulama of our times have made ijma that it is permissible to kill them. Those who doubt their kufr becomes a kafir. According to Imam al-Azam Imam Abu Hanifah, Imam Sufyan and Imam Auwzai if they repent then their repentance will be accepted like the rest of the kuffar. Imam Malik, Imam Shafi, Imam Ahmad bin Hanbal and Layth bin Saad said that their repentance will not be accepted..."
(Rasa'il ibn 'Abidin, 1:368:369)
Verdict of Imam Bukhari
Imam Bukhari (may Allah have mercy on him) writes in his book,
“I don’t see any difference between praying Salat behind a Jahmi or a Rafidi and a Christian or a Jew [because these two sects are kafir like Jews and Christians, even if they refer to themselves as Muslim]. They [Jahmis/Rawfid] are not to be greeted, nor are they to be visited, nor are they to be married, nor is their testimony to be accepted, nor are their sacrifices (zabiha) to be eaten.”
(Khalq Af'al al-Ibad, p.13)