Musnad Ahmad 1153:2094

Sulayman ibn Dawud—meaning Abu Dawud al-Tayalisi—narrated to us, Shu'bah narrated to us, from Musa ibn Abi 'Aishah, he said: I heard 'Ubayd Allah ibn 'Abd Allah ibn 'Utbah narrating from 'Aishah, that the Messenger of Allah (peace and blessings of Allah be upon him) ordered Abu Bakr to lead the people in prayer during his illness in which he died. The Messenger of Allah (peace and blessings of Allah be upon him) was in front of Abu Bakr, praying with the people while sitting, and Abu Bakr was praying with the people, and the people were behind him.