Musnad Ahmad 33:666

'Abd al-Samad narrated to us, my father narrated to me, Kathir ibn Shinzhir narrated to us, 'Ata' ibn Abi Rabah narrated to us, from Jabir ibn 'Abd Allah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) sent me on an errand, so I went, then I returned having fulfilled it. I came to the Prophet (peace and blessings of Allah be upon him) and greeted him, but he did not return the greeting to me. He said: There fell into my heart that which only Allah knows. He said: I said, "Perhaps the Messenger of Allah (peace and blessings of Allah be upon him) is upset with me because I was slow." So I greeted him, and he did not return the greeting to me. There fell into my heart that which only Allah knows, more intense than the first time. Then I greeted him, and he returned the greeting to me and said: "Indeed, nothing prevented me from returning your greeting except that I was praying." He was upon his mount, facing a direction other than the Qibla.