مُوَطَّأُ مَالِكٍ
Muwatta Malik 342
Shortening the Prayer
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّهُ كَانَ يُسَافِرُ إِلَى خَيْبَرَ فَيَقْصُرُ الصَّلاَةَ .
Yahya related to me from Malik from Nafi from Ibn Umar that he used to travel to Khaybar and he would shorten the prayer.
Arabic/English book reference: Book 9, Hadith 14