Musnad Abu Ya'la al-Mawsili 118:32

Ishaq ibn Abi Isra'il and others narrated to us from Ibn al-Mubarak, Abu Bakr narrated to us, Yazid ibn Harun narrated to us, from Sufyan ibn Husayn, from al-Zuhri, from Anas, who said: When the Messenger of Allah (may Allah bless him and grant him peace) became ill with the illness from which he died, Bilal came to him and called the Adhan for prayer. He said, "O Bilal, you have announced it; so whoever wishes, let him pray, and whoever wishes, let him leave." He said, "O Messenger of Allah, 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 (may Allah bless him and grant him peace), and we looked at him as if he were a white sheet, wearing a black *khamisa* (garment). Abu Bakr thought that he wanted to come out, so he stepped back. The Messenger of Allah (may Allah bless him and grant him peace) signaled to him to pray where he was. So Abu Bakr prayed, and we did not see the Messenger of Allah (may Allah bless him and grant him peace) again until he died on that day.