مُوَطَّأُ مَالِكٍ
Muwatta Malik 344
Shortening the Prayer
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّهُ كَانَ يُسَافِرُ مَعَ ابْنِ عُمَرَ الْبَرِيدَ فَلاَ يَقْصُرُ الصَّلاَةَ .
Yahya related to me from Malik from Nafi that he used to travel one mail-stage with Ibn Umar, and he would not shorten the prayer.
Arabic/English book reference: Book 9, Hadith 16