Original post by MENDACIUMSalamunalaykum,
I will begin with this hadith. This was shortly spoken by Muhammed pbuh within a short period before his death, infront of a very large number of muslims. The source is considered among one of the authentic books in the ahle sunnah:
Then the Messenger of Allah continued: "Do I not have more right over the believers than what they have over themselves?" People cried and answered: "Yes, O' Messenger of God." Then Prophet (PBUH) held up the hand of Ali and said: "Whoever I am his leader (Mawla), Ali is his leader (Mawla). O' God, love those who love him, and be hostile to those who are hostile to him."
Sahih Tirmidhi, v2, p298, v5, p63
As you only accept hadiths in the six authentic books , and those of authentic commentators/scholars, i will only quote those. I will infact, begin by allowing the words of Hadrath Umar , as narrated in Al Bukhar(most people say it is the most authentic book of hadith in the ahle sunnah, though some say it is Muslim):
When Ali a.s. was burying Muhammed pbuh, Hadrath Abubakr and Hadrath Umar went to saqifah many miles from the mosque of Muhamed pbuh, to try and debate as to who should be the next caliph.
Hadrath Umar himself states in Al Bukhari:
"One should not deceive oneself by saying that the pledge of allegiance given to Abu Bakr was given suddenly and it was successful.
Imam Ali a.s. opposed Hadrath Abubakr, again narrated by Hadrath Umar in Al Bukhari:
And no doubt after the death of the Prophet we were informed that the Ansar disagreed with us and gathered in the shed of Bani Sa'da. 'Ali and Zubair and whoever was with them, opposed us, while the emigrants gathered with Abu Bakr.
Hadrath Umar admits in Saqifah when the Ansar were disagreeing with the group of Hadrath Abubakr as to who should be the caliph, and when there was chaos, he took the chance among the chaos -without the consultation of the muslims even at saqifah, to quickly pledge allegiance to Hadrath Abubakr, in Al Bukhari:
Then there was a hue and cry among the gathering and their voices rose so that I was afraid there might be great disagreement, so I said, 'O Abu Bakr! Hold your hand out.' He held his hand out and I pledged allegiance to him, and THEN all the emigrants gave the Pledge of allegiance and so did the Ansar afterwards.
Hadrath Umar himself states, if anyone is to do this again, to give them capital punishment, in Al Bukhari:
So if any person gives the Pledge of allegiance to somebody (to become a Caliph) WITHOUT consulting the other Muslims, then the one he has selected should NOT be granted allegiance, lest both of them should be killed."
Despite opposing Hadrath Abubakr, Ali a.s. did not take out his sword and thus cause a civil war, because the muslims would become fragmented and weak, and outside enemies would take the chance to destroy them.