It is narrated from Salim bin Abdullah that Abdullah bin Umar (may Allah be pleased with them) traveled towards "Dhat al-Nusub," and during this journey, he shortened the prayer. Imam Malik (may Allah have mercy on him) says: There is a distance of four barid between Madinah and Dhat al-Nusub.
Hadith Referenceمسند الإمام الشافعي / كتاب الصلاة / 348
Hadith Takhrijأخرجه مالك في الموطأ: الصلاة، باب ما يجب فيه قصر الصلاة - وابن أبي شيبة: 8136 - والبيهقي: 3/ 136 وفي معرفة السنن والآثار له (1582).