مُوَطَّأُ مَالِكٍ
Muwatta Malik 355
Shortening the Prayer
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 .
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."
Arabic/English book reference: Book 9, Hadith 26