Abu Saeed Khudri reported,
When the Messenger of Allah ﷺ passed away, the speakers of the Ansar rose to address the gathering. Some of them said, "People of the Muhajirun, whenever the Messenger of Allah ﷺ appointed one of you to a position, he would pair with him a man from among us. We therefore think that two men should lead this affair, one from you and one from us."The speakers of the Ansar continued speaking along these lines.Then Zayd bin Thabit stood up and said, "The Messenger of Allah ﷺ was from the Muhajirun, and the leader can only be from the Muhajirun. We will be his helpers just as we were the helpers of the Messenger of Allah ﷺ."Abu Bakr then stood and said, "May Allah reward you well, people of the Ansar, as a community, and may He keep your spokesman firm." Then he said, "By Allah, had you done otherwise, we would not have made peace with you."
Azami 9366
Sahihعن أبي سعيد الخدري قال
لما توفي رسول الله ﷺ قام خطباء الأنصار، فجعل منهم من يقول: يا معشر المهاجرين، إن رسول الله ﷺ كان إذا استعمل رجلا منكم قرن معه رجلا منا، فنرى أن يلي هذا الأمر رجلان: أحدهما منكم والآخر منا. قال: فتتابعت خطباء الأنصار على ذلك، قال: فقام زيد بن ثابت فقال: إن رسول الله ﷺ كان من المهاجرين، وإنما الإمام يكون من المهاجرين، ونحن أنصاره كما كنا أنصار رسول الله ﷺ فقام أبو بكر فقال: جزاكم الله خيرا من حي يا معشر الأنصار وثبت قائلكم ثم قال: والله لو فعلتم غير ذلك لما صالحناكم.
Narrated by Ahmad (21617), Tabarani in al-Kabir (5/122), and Hakim (3/76), all of them from the hadith of Affan bin Muslim, who said, Waheeb bin Khalid told us, Dawud bin Abu Hind told us, from Abu Nadra, from Abu Saeed Khudri, and he mentioned it.Hakim added, "When Abu Bakr sat on the pulpit, he looked at the faces of the people and did not see Ali, so he asked about him. Some men from the Ansar stood up and brought him. Abu Bakr said, 'Cousin of the Messenger of Allah ﷺ and his son-in-law, did you intend to break the unity of the Muslims?' Ali said, 'No blame upon you, Caliph of the Messenger of Allah ﷺ.' Then he gave him his pledge of allegiance. Then he did not see Zubayr bin Awwam, so he asked about him until they brought him. He said, 'Son of the Messenger of Allah's paternal aunt ﷺ and his close companion, did you intend to break the unity of the Muslims?' Zubayr said the same as Ali had said, 'No blame upon you, Caliph of the Messenger of Allah ﷺ.' Then both of them gave him their pledge of allegiance."Its chain of transmission is sahih. Hakim said, "This is a sahih hadith meeting the criteria of the two Shaykhs (Bukhari and Muslim)," and Dhahabi passed over it in silence.Regarding the statement "then both of them gave him their pledge of allegiance," this pledge of allegiance by Ali bin Abu Talib remained hidden from most people, because he did not take part in Abu Bakr's political life during her lifetime. But after the death of Fatimah 
, he began to take part with Abu Bakr in political matters and pledged allegiance to him a second time. This second pledge of allegiance is mentioned in the account of the distribution of the spoils of Khaybar in the Book of Military Expeditions.Ibn Kathir related in al-Bidayah wal-Nihayah (8/90), from Abu Ali the Hafiz, Husayn bin Ali, who said, "I heard Muhammad bin Ishaq bin Khuzayma say, 'Muslim bin Hajjaj came to me and asked me about this hadith, so I wrote it down for him on a slip of paper and read it to him.' He said, 'This hadith is worth a badanah (a sacrificial camel).' I said, 'It is not worth a badanah, rather it is worth a badrah.'" A badrah is a purse containing a certain amount of money used in transactions, and it is presented as a gift.