Musnad Ahmad 33:473

Yunus ibn Muhammad and Hujayn narrated to us, they said: Layth narrated to us, from Abu al-Zubayr, from Jabir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) sent me for a need, then I caught up with him and greeted him, but he gestured to me. When he finished, he called me and said: "You greeted me just now while I was praying." He was facing towards the east at that time.