Maliks Muwatta Book 9, Hadith Number 25.

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

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.”