Musnad Ahmad 33:790

'Affan narrated to us, Yazid bin Ibrahim narrated to us, Abu al-Zubayr narrated to us, from Jabir bin 'Abdullah al-Ansari, that the Prophet, peace and blessings of Allah be upon him, sent him for some need of his. He said: So he came while the Prophet, peace and blessings of Allah be upon him, was praying upon his riding beast. He said: So he greeted him, and he remained silent. He greeted him, and he remained silent. He greeted him, and he remained silent—three times. He said: So he said to him when he finished: "Indeed, nothing prevented me from returning your greeting except that I was praying." He said: So he prayed wherever his riding beast faced.