Yahya related to me from Malik that he had heard that al-Qasimibn Muhammad and Urwa ibn az-Zubayr and Abu Bakr ibn Abd ar Rahmanused to pray nawafil when travelling.Yahya said that Malikwas asked about voluntary prayers when travelling and he said, "Thereis no harm in them, either at night or during the day. I have heardthat some of the people of knowledge used to do that."
USC-MSA web (English) reference: Book 9, Hadith 25
Arabic reference: Book 9, Hadith 354