Muwatta Imam Malik Book of Shortening the Prayer, Hadith 26

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."

وَحَدَّثَنِي عَنْ مَالِكٍ، قَالَ بَلَغَنِي عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَرَى ابْنَهُ عُبَيْدَ اللَّهِ بْنَ عَبْدِ اللَّهِ يَتَنَفَّلُ فِي السَّفَرِ فَلاَ يُنْكِرُ عَلَيْهِ .