Musnad Abu Ya'la al-Mawsili 109:258

Abu Khaythama narrated to us, Abd al-Samad narrated to us, Kathir narrated to us, Ata’ narrated from Jabir, who said: The Messenger of Allah (peace be upon him) sent me [on an errand], so I went, and then I returned having fulfilled it. I came to the Prophet (peace be upon him) and greeted him, but he did not return the greeting to me. There occurred in my soul [a feeling] that only Allah knows. I said: "Perhaps the Messenger of Allah (peace be upon him) is upset with me because I was slow." So I greeted him [again], and he did not return the greeting to me. There occurred in my soul [a feeling] more intense than the first. Then I greeted him [a third time], and he returned the greeting to me and said: "Nothing prevented me from returning your greeting except that I was praying." And he was upon his mount, facing a direction other than the Qibla.