Musnad Ahmad 1153:1240

Shababah narrated to us, Shu'bah narrated to us, from Sa'd ibn Ibrahim, from 'Urwah ibn al-Zubayr, from 'Aishah, she said: The Messenger of Allah (peace and blessings of Allah be upon him) said during his illness in which he died: "Command Abu Bakr to lead the people in prayer." 'Aishah said: "Indeed, Abu Bakr is a soft-hearted man, and whenever he stands in your place, tenderness will overcome him." The Prophet (peace and blessings of Allah be upon him) said: "You are the companions of Yusuf. Command Abu Bakr to lead the people in prayer." So Abu Bakr led the prayer, and the Prophet (peace and blessings of Allah be upon him) prayed behind him while sitting.