Hadith 503440
Chapter 9: Shortening the Prayer - كتاب قصر الصلاة فى السفر
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.
Reference Muwatta Malik 503440
USC-MSA web (English) reference : Book 9, Hadith 16