Musnad Ahmad 33:440

Abu Ahmad narrated to us, Sufyan narrated to us, from Abu al-Zubayr, from Jabir, who said: The Prophet, peace and blessings of Allah be upon him, sent me for a need. I came while he was praying upon his riding beast facing the east, and he was gesturing with a gesture; the prostration was lower than the bowing. I greeted him, and when he finished, he said: "What did you do regarding such and such need? I was praying."