Muwatta Imam Malik Book of Shortening the Prayer, Hadith 25

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."

وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ الْقَاسِمَ بْنَ مُحَمَّدٍ، وَعُرْوَةَ بْنَ الزُّبَيْرِ، وَأَبَا، بَكْرِ بْنَ عَبْدِ الرَّحْمَنِ كَانُوا يَتَنَفَّلُونَ فِي السَّفَرِ . قَالَ يَحْيَى وَسُئِلَ مَالِكٌ عَنِ النَّافِلَةِ فِي السَّفَرِ فَقَالَ لاَ بَأْسَ بِذَلِكَ بِاللَّيْلِ وَالنَّهَارِ وَقَدْ بَلَغَنِي أَنَّ بَعْضَ أَهْلِ الْعِلْمِ كَانَ يَفْعَلُ ذَلِكَ .