1) Have I not ascertained its reliability by Sunnis, I would not have mentioned it to you. Yet Ibn Jarir and Imam Abu Ja’far al-Iskafi have taken its authenticity for granted.1 Several other critics have also considered it authentic. It is sufficient proof for its authenticity the fact that it is reported by the reliable authorities upon whose accuracy the authors of sahih books rely unhesitatingly. Refer to page 111, Vol. 1, of Ahmad's Musnad, where you will read this hadith as narrated by Aswad ibn ‘Amir2 from Sharik,3 al-A’mash,4 Minhal,5 ‘Abbad ibn ‘Abdullah al-Asadi,6 from ‘Ali (as) chronologically.
Each one of these men in the chain of narrators is an authority in his own right, and they all are reliable traditionists according to the testimony of the authors of the sahih books without any dispute. Al-Qaysarani has mentioned them in his book Al-Jami’ Bayna Rijal al-Sahihain. There is no doubt that this hadith is authentic, and the narrators report it from various ways each one of which supports the other.
2) The reason why both shaykhs [Bukhari and Muslim], and their likes, have not quoted this hadith is due to the fact that it did not agree with their own personal views regarding the issue of succession. This is why they have rejected a great deal of authentic texts for fear the Shi’as may use them as pretexts; therefore, they hid the truth knowingly.
There are many Sunni shaykhs, may Allah forgive them, who have likewise hidden such texts, and they have in their method of hiding a well known history written down by al-Hafiz ibn Hajar in his Fath Al-Barari. Al-Bukhari has assigned a special chapter for this theme at the conclusion of his chapter on "Al-’Ilm," in Vol. 1, page 25, of his Sahih, subtitled "A Chapter on Those Who Recognized the Knowledge of some People Rather than that of Others."
3) Whoever knows the way al-Bukhari thought, his own attitudes towards the Commander of the Faithful (as), and towards all Ahl al-Bayt (as), will come to know that Bukhari's pen falls short of narrating texts regarding them, and his ink dries up before recounting their attributes. He will not be surprised to see him rejecting this particular hadith as well as others similar to it; therefore, we seek refuge with Allah, the Almighty, the Sublime, and peace be with you.