Maliks Muwatta Book 9, Hadith Number 26.

Section : Voluntary Prayers while Travelling, by Day and at Night, and Praying on a Riding Beast.

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.”