Musannaf Ibn Abi Shayba 3:4794

Jarir narrated to us, from Abu Hazim, from Sahl ibn Sa'd, who said: There was a dispute among the Ansar. The Prophet, peace and blessings of Allah be upon him, came to them to reconcile between them. Then Abu Bakr came to lead the people in prayer. He said: So he [the Prophet] prayed behind Abu Bakr.