Muwatta Malik Book 9, Hadith 26
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.
Yahya related to me from Malik from Ibn Shihab that Safwan said, "Abdullah ibn Umar used to come and visit Abdullah ibn Safwan and he would pray two rakas with us, and when he went away we would stand and complete the prayer."
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."