Musnad Ahmad 33:507

Hassan narrated to us, Ibn Lahi'ah narrated to us, Abu al-Zubayr narrated to us, he said: I asked Jabir, "Did you see the Messenger of Allah, may Allah bless him and grant him peace, pray while riding?" He said: "Yes. Then a man came to him who had bought a she-camel to pray to Allah, the Almighty and Majestic, upon it. He spoke to the Messenger of Allah, may Allah bless him and grant him peace, and the Messenger of Allah, may Allah bless him and grant him peace, remained silent until he finished the prayer, then he prayed for him."