Musnad Abu Ya'la al-Mawsili 154:148

Abu Khaythama narrated to us, 'Abd al-Malik ibn 'Amr narrated to us, 'Isa ibn Hafs narrated to me, my father informed me: That he was with Ibn 'Umar on the road to Makkah, and he prayed with them, then he turned toward his mount, and we turned with him. He looked back and saw people praying. He said, "What are these people doing?" I said, "They are performing voluntary prayers (tasbih)." He said, "If I were to perform voluntary prayers, I would have completed my prayer. O son of my brother, I accompanied the Messenger of Allah (may Allah bless him and grant him peace), and he did not exceed two rak'ahs during travel until Allah took his soul. Then I accompanied Abu Bakr, and he did not exceed two rak'ahs. Then I accompanied 'Umar, and he did not exceed two rak'ahs. Then I accompanied 'Uthman, and he did not exceed two rak'ahs until Allah took his soul." And Allah said: "Indeed, there is for you in the Messenger of Allah an excellent pattern" (Al-Ahzab).