Musnad Ahmad 25:1496

Waki’ narrated to us, Isra’il narrated to us, from Abu Ishaq, from Arqam ibn Sharhabil, from Ibn Abbas, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) became ill with the illness in which he died, he was in the house of Aisha. He said: "Call Ali for me." Aisha said: "Shall we call Abu Bakr for you?" He said: "Call him." Hafsa said: "O Messenger of Allah, shall we call Umar for you?" He said: "Call him." Umm al-Fadl said: "O Messenger of Allah, shall we call al-Abbas for you?" He said: "Call him." When they gathered, he raised his head and did not see Ali, so he remained silent. Umar said: "Rise from the Messenger of Allah (peace and blessings of Allah be upon him)." Then Bilal came to inform him of the prayer, and he said: "Command Abu Bakr to lead the people in prayer." Aisha said: "Abu Bakr is a soft-hearted man, and whenever the people do not see you, they weep. If you were to command Umar to lead the people in prayer..." So Abu Bakr went out and led the people in prayer. The Prophet (peace and blessings of Allah be upon him) felt a lightness in himself, so he went out being supported between two men, his feet dragging on the ground. When the people saw him, they glorified [Allah] for Abu Bakr. He [Abu Bakr] went to step back, but he [the Prophet] gestured to him: "Stay in your place." The Prophet (peace and blessings of Allah be upon him) came until he sat down. He said: Abu Bakr stood to his right, and Abu Bakr would follow the Prophet (peace and blessings of Allah be upon him) in prayer, and the people would follow Abu Bakr. Ibn Abbas said: The Prophet (peace and blessings of Allah be upon him) took up the recitation from where Abu Bakr had reached. He died in that illness of his, peace be upon him. Waki’ said once: "And Abu Bakr would follow the Prophet (peace and blessings of Allah be upon him) in prayer, and the people would follow Abu Bakr."