Muwatta Malik Book 9, Hadith 28
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."
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."
Yahya related to me from Malik from Amr ibn Yahya al-Mazini from Abu'l-Hubab Said ibn Yasar that Abdullah ibn Umar said, "I saw the Messenger of Allah, may Allah bless him and grant him peace, praying on a donkey while heading towards Khaybar."