Musnad Ahmad 32:1150

Yazid narrated to us, Sufyan—meaning Ibn Husayn—informed us, from al-Zuhri, from Anas, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) became ill with the illness in which he passed away, Bilal came to him to notify him of the prayer. He said after two times: "O Bilal, you have conveyed [the message]; whoever wishes, let him pray, and whoever wishes, let him leave it." Bilal returned to him and said: "O Messenger of Allah, may my father and mother be sacrificed for you, who shall lead the people in prayer?" He said: "Command Abu Bakr to lead the people in prayer." When Abu Bakr stepped forward, the curtains were raised from the Messenger of Allah (peace and blessings of Allah be upon him). He said: We looked at him as if he were a white sheet, wearing a *khamisah* (a garment). Abu Bakr went to step back, thinking that he [the Prophet] intended to come out for the prayer, but the Messenger of Allah (peace and blessings of Allah be upon him) signaled to Abu Bakr to remain and pray. So Abu Bakr led the people in prayer, and we did not see him [the Prophet] after that.