Musnad Ahmad 788:214

Husayn ibn Ali narrated to us, from Za'idah, from Abd al-Malik ibn Umayr, from Abu Burdah ibn Abi Musa, from Abu Musa, who said: The Messenger of Allah (peace and blessings of Allah be upon him) became ill, and his illness intensified. He said: "Command Abu Bakr to lead the people in prayer." Aisha said: "O Messenger of Allah, Abu Bakr is a tender-hearted man. Whenever he stands in your place, he will not be able to lead the people in prayer." He said: "Command Abu Bakr to lead the people in prayer, for you are the companions of Yusuf." So the messenger came to him, and Abu Bakr led the people in prayer during the lifetime of the Messenger of Allah (peace and blessings of Allah be upon him).