Yahya related to me from Malik that he had heard that al-Qasim ibn Muhammad and Urwa ibn az-Zubayr and Abu Bakr ibn Abd ar Rahman used to pray nawafil when travelling.

Yahya said that Malik was asked about voluntary prayers when travelling and he said, "There is no harm in them, either at night or during the day. I have heard that some of the people of knowledge used to do that."
Muwatta Malik Book 9, Hadith 27
← Previous Hadith Yahya related to me from Malik from Nafi that Abdullah ibn Umar never used to pray anything with the fard prayer, either before it or after it, while travelling, except in the depths of the night. He would pray on the ground or on his mount, whichever way it was facing.

Muwatta Malik Book 9, Hadith 26 Next Hadith → Yahya related to me that Malik said, "I have heard from Nafi that Abdullah ibn Umar used to see his son, Ubaydullah ibn Abdullah, doing voluntary prayers on a journey,and he would not disapprove of it."

Muwatta Malik Book 9, Hadith 28