It is narrated from Ibn Umar (may Allah be pleased with them both) that he went towards Dhat al-Nusub and performed shortened (qasr) prayer. Imam Malik (may Allah have mercy on him) says: And this is at a distance of four barid.
Hadith Referenceمسند الإمام الشافعي / كتاب الصلاة / 352
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).