Muwatta Malik Book 9, Hadith 15
Yahya related to me from Malik from Nafi from Ibn Umar that he
used to travel to Khaybar and he would shorten the prayer.
Yahya related to me from Malik from Nafi from Salim ibn Abdullah that Abdullah ibn Umar rode to Dhat an-Nusub and shortened the prayer on the journey.
Malik said, "There are four mail-stages between Dhat an-Nusub and Madina."
Yahya related to me from Malik from Ibn Shihab from Salim ibn Abdullah that Abdullah ibn Umar used to shorten the prayer when he travelled for a whole day.