Musnad Abu Ya'la al-Mawsili 109:231

Abu Khaythama narrated to us, Rawh narrated to us, Zakariyya and Hisham ibn Abi Abd Allah narrated to us, they said: Abu al-Zubayr informed us, from Jabir, who said: The Messenger of Allah (may Allah bless him and grant him peace) sent me for a need. I returned while he was upon his riding beast, so I greeted him, but he did not return anything to me. Zakariyya added: Then I greeted him, but he did not return anything to me. Then their narration agreed after that: I saw him bowing and prostrating, so I moved away from him. Then he said: "What did you do regarding your need?" I said: "I did such and such." He said: "Nothing prevented me from returning your greeting except that I was praying." Zakariyya added: When he finished his prayer, he called me, returned my greeting, and said: "I was praying."