Musnad Ahmad 33:671

'Abd al-Samad and Kathir ibn Hisham narrated to us, they said: Hisham 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 of his. I returned to him while he was upon his riding beast, and I greeted him, but he did not return the greeting to me. I saw him bowing and prostrating, so I moved away from him. Then he said to me: "What did you do regarding your need?" I said: I did such and such. He said: "Indeed, nothing prevented me from returning your greeting except that I was praying."