Muwatta Malik, Book 9, Hadith 341 (In-Book)

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."

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 ‏.‏

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also referenced under: Book 9, Hadith 13 (USC-MSA)