Hadith 503410
Chapter 9: Shortening the Prayer - كتاب قصر الصلاة فى السفر
حَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ سَالِمِ بْنِ عَبْدِ اللَّهِ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، رَكِبَ إِلَى ذَاتِ النُّصُبِ فَقَصَرَ الصَّلاَةَ فِي مَسِيرِهِ ذَلِكَ . قَالَ مَالِكٌ وَبَيْنَ ذَاتِ النُّصُبِ وَالْمَدِينَةِ أَرْبَعَةُ بُرُدٍ .
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."
Reference Muwatta Malik 503410
USC-MSA web (English) reference : Book 9, Hadith 13