Yahya related to me from Malik from Nafi from Salim ibn Abdullahthat Abdullah ibn Umar rode to Dhat an-Nusub and shortened the prayeron the journey.Malik said, "There are four mail-stagesbetween Dhat an-Nusub and Madina."
USC-MSA web (English) reference: Book 9, Hadith 13
Arabic reference: Book 9, Hadith 341