Musnad Ahmad 33:1049

Ishaq ibn Isa narrated to us, he said: Hammad—meaning ibn Zayd—narrated to us, from Kathir ibn Shanzir, from Ata ibn Abi Rabah, from Jabir ibn Abdullah, who said: The Prophet, peace and blessings of Allah be upon him, sent me on an errand. When I returned, I greeted him, but he did not return the greeting to me. When he finished, I said: "O Messenger of Allah, I greeted you, but you did not return the greeting to me." He said: "I was praying." And he was upon his mount, facing a direction other than the Qibla.