The Twelvers claim that the Prophet ﷺ appointed ‘Ali Ibn Abi Taleb (رضي الله عنه) as his successor. Imam Fakhr Al-Deen Al-Razi argues against this claim in his Ma’alim Uusul Al-Deen. Al-Razi writes:
|The Twelvers claim: the Prophet ﷺ explicitly appointed ‘Ali (رضي الله عنه) as his rightful successor. And that this explicit proclamation from the Prophet is so clear, that it leaves no room for alternative interpretation. Whereas the other groups claim that such a proclamation never happened.
We offer the following in response to the Twelver claim.
First: a proclamation designating the legitimate successor would have been an extremely important one. Such an important event ought to have been reported from the Prophet ﷺ by multiple witnesses, reaching both the Twelvers and their opponents. And since a report pertaining to this proclamation has reached neither the Fuqaha’ nor the Muhaditheen, we come to know that this claim is a lie.
Second: if such a proclamation occurred, then either:
The first is false. For most of the Companions did not seek to become Caliph, and the few who did, did not do so out of selfish desire. Rather, they extolled the Prophet ﷺ, and believed disobeying him entailed harsh punishment. And humans do not freely choose to receive punishment without impetus, especially when there are reasons to side with Ali (رضي الله عنه) [over those who allegedly oppressed him]. Amongst those reasons:
Firstly: that ‘Ali was extremely brave. And Abu Bakr (رضي الله عنه) was extremely weak according to the Rawafid.
Secondly: that those who were loyal to ‘Ali were of high nobility. Since Fatima, Al-Hassan, Al-Hussain, and Al-‘Abbas were all with him (رضي الله عنهم). As well as Abu Sufyan, the leader of Banu Ummayah, who strongly disapproved of Abu Bakr (رضي الله عنهما), and encouraged ‘Ali to take the role of Caliph from him. And Al-Zubayr (رضي الله عنه) with his bravery, who went so far as to threaten Abu Bakr with his sword.
Thirdly: that the Ansar (رضي الله عنهم) sought leadership for someone from amongst them. And Abu Bakr was the one who prevented them from taking it. So if the Prophet ﷺ had really appointed ‘Ali as his legitimate successor, the Ansar would have told Abu Bakr: “we wanted to unrightfully take this position for ourselves, but since you prevented us, we will also prevent you from unrightfully taking this position for yourself, and we will side with ‘Ali.” For if Abu Bakr and the Ansar competed for leadership, and if the Prophet ﷺ designated ‘Ali to be his successor, then the Ansar would not have hesitated to side with ‘Ali when their opponent prevented them from taking the position.
Given all of the above, it is established that ‘Ali would have had the upper-hand [and thus become the Caliph] had the Prophet ﷺ really made such a proclamation. But since this did not occur, we come to know that there is no basis for the Twelver claim.
As for the second choice, namely: that the Prophet ﷺ did not designate ‘Ali to be his successor in the presence of those who would mass transmit it, then it is farfetched for many reasons.
Firstly: a singular report cannot be used as a premise in a [decisive] argument. This is especially true according to them [the Twelvers], since they do not consider it a way to knowledge.
Secondly: this is tantamount to accusing the Prophet ﷺ of betrayal. As it would mean that he chose to not reveal extremely important information. Thus, what they [the Twelvers] claim must be false.
Third: that ‘Ali (رضي الله عنه) had narrated much of what the Prophet ﷺ told him privately in person. And the proclamation that the Twelvers attribute to the Prophet ﷺ is not among those narrations. If the Prophet ﷺ actually made such a statement, then ‘Ali would have certainly narrated it over anything else.
And they [the Twelvers] argued that the Shi’a, while being numerous and dispersed, all narrate this proclamation from the Prophet ﷺ.
The response: it is famously known that the one who fabricated this report is Ibn Al-Rawandi. Furthermore, there is a motivation for the Shi’a to spread this lie [which is why it spread amongst them].
– Al-Razi, Ma’alim Usul Al-Deen (pg. 149)
 Entailing that Abu Bakr, Umar, and Uthman (رضي الله عنهم), all be illegitimate Caliphs. This is also false for the reasons mentioned here.
 If we suppose that Abu Bakr was of poor character, as some of the Twelvers claim he was, then most of the Companions should have sided with someone like ‘Ali over him. Even more so if the Prophet ﷺ also publicly designated ‘Ali has his rightful successor.
 The Arabs were mindful of lineages when appointing their leaders. And ‘Ali (رضي الله عنه), as well as those who supported him, were close relatives of the Prophet ﷺ, from Banu Hashim. So if the Prophet ﷺ had really appointed ‘Ali has his successor, then the Arabs would have most certainly supported him against Abu Bakr, who was a more distant relative of the Prophet.
 I could not find a reliable reference for this. But Al-Tilmisani reports, without a chain, on page 678 of his commentary on Al-Razi’s Ma’alim Usul Al-Deen:
“In reference to Abu Sufyan’s saying to ‘Ali (رضي الله عنه): ‘how can you leave this matter to the least noble of the tribes [from Quraysh]?’. And ‘Ali (رضي الله عنه) responded: ‘you caused us trouble before your Islam, and now you want to cause us trouble while you’re a Muslim!?’”
 I could not find a reliable reference for this incident either.
Nevertheless, the Imam’s argument holds. Since the Twelvers do accept that Fatima, Al-Hassan, Al-Hussain, and Al-‘Abbas, all sided with ‘Ali against Abu Bakr. Further still, some of the Twelvers claim that the Prophet ﷺ did not praise Abu Bakr, and that those reports attributing praise of Abu Bakr to the Prophet ﷺ are fabrications by later Sunni narrators.
With the above in mind we say: if we suppose that the Prophet ﷺ did not praise Abu Bakr, and given the status of ‘Ali and those who sided with him in the eyes of the Prophet ﷺ and the greater Muslim community, then there is all the more reason for the Muslims to support ‘Ali over Abu Bakr, had the Prophet ﷺ appointed ‘Ali as his successor.
 In reference to the famous dispute that occurred in the Saqifa of Bani Sa’ida, reported in Sahih Al-Bukhari, Hadith 6830, and other works.
 In essence, the opponent would be claiming that the Prophet ﷺ kept this information a secret between himself and a select few of those around him. Even though he ﷺ could have spared this nation much disagreement, and much bloodshed, by simply announcing this publicly for all to hear. And this would be especially problematic for the Twelvers, who believe in Al-Tahsin wa Al-Taqbih Al-‘Aqliayn.
 Additionally, the Twelver must demonstrate that this alleged proclamation was mass transmitted from the time of the Prophet ﷺ. It is not enough for it to have only been mass transmitted from later-generation narrators, as that would only be proof that those later narrators claimed that the Prophet ﷺ made this proclamation, not proof that the Prophet ﷺ actually made this proclamation.