Musnad Ahmad 1153:630

'Affan narrated to us, he said: Hammad ibn Salamah narrated to us, from Hisham ibn 'Urwah, from his father, from 'Aishah, that the Prophet, peace and blessings of Allah be upon him, said during his illness: "Command Abu Bakr to lead the people in prayer." 'Aishah said to Hafsah: "Abu Bakr is a tender-hearted man, and if he stands in your place, the people will not hear him because of his weeping." He said: "Command him to lead the people in prayer." She said: She repeated it to him several times, and each time he said: "Command Abu Bakr to lead the people in prayer." Then he said on the third time: "Leave me, for you are the companions of Yusuf. Let Abu Bakr lead the people in prayer."