Musnad Ahmad 788:215

Abu Sa'id, the freed slave of Banu Hashim, narrated to us, saying: Za'idah narrated to us, saying: 'Abd al-Malik—meaning Ibn 'Umayr—narrated to us, from Abu Burdah ibn Abi Musa, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) became ill, so he said: "Command Abu Bakr to lead the people in prayer." He then mentioned the rest of the hadith.