سنن ابن ماجه

It was narrated from ‘Isa bin Hafs bin ‘Asim bin ‘Umar bin Khattab that his father told him: “We were with Ibn ‘Umar on a journey, and he led us in prayer. Then we finished with him and he finished turning around, and saw some people praying. He said: ‘What are these people doing?’ I said: ‘Glorifying Allah.’* He said: ‘If I wanted to glorify Allah (perform voluntary prayer) I would have completed my prayer. O son of my brother! I accompanied the Messenger of Allah (ﷺ) and he never prayed more than two Rak’ah when he was traveling, until Allah took his soul. Then I accompanied Abu Bakr and he never prayed more than two Rak’ah (when he was traveling), until Allah took his soul. Then I accompanied ‘Umar and he never prayed more than two Rak’ah, until Allah took his soul. Then I accompanied ‘Uthman and he never prayed more than two Rak’ah, until Allah took his soul. Allah says: ‘Indeed in the Messenger of Allah (Muhammad (ﷺ)) you have a good example to follow.’” [33:21]
* Meaning, they were offering voluntary prayer.