Isa bin Hafs bin Asim bin Umar bin al-Khattab narrates from his father that he said: I was with Ibn Umar (may Allah be pleased with them both) on the road to Makkah. He led us in two rak'ahs, then turned and saw people standing. He asked: "What are these people doing?" I said: "They are praying voluntary prayers." He said: "O my nephew! If I were going to pray voluntary prayers, I would have completed my prayer. I was with the Messenger of Allah (peace and blessings be upon him) on a journey, and he (peace and blessings be upon him) did not pray more than two rak'ahs, until Allah took his (peace and blessings be upon him) soul. I was with Abu Bakr (may Allah be pleased with him), Umar (may Allah be pleased with him) and Uthman (may Allah be pleased with him), and they did not pray more than two rak'ahs on a journey, until Allah took their souls." Allah Almighty says: "Indeed, in the Messenger of Allah you have an excellent example." [Surah al-Ahzab: 21]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5507
Nafi' narrates from Sayyiduna Ibn Umar (may Allah be pleased with them both) that on a journey he would not pray anything with the obligatory prayers, neither before them nor after them; he would only pray the night prayer (tahajjud), and that too on his mount, whichever direction it faced.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5508