সুনানে ইবনে মাজাহ ১০৭০

It was narrated from Mujahid, Sa’eed bin Jubair, ‘Ata’ bin Abi Rabah and Tawus that Ibn ‘Abbas told them that the Messenger of Allah (ﷺ) used to combine the Maghrib and ‘Isha’ when traveling, although there was nothing to make him hurry and no enemy pursuing him, and he was not afraid of anything

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

It was narrated that Ibn ‘Abbas said:“Allah enjoined the prayer upon the tongue of your Prophet (ﷺ): Four Rak’ah while a resident and two Rak’ah when traveling.”