Salim bin Abdullah narrates from his father, Abdullah bin Umar (may Allah be pleased with them both), that he went to the place Dhat al-Nusub and shortened the prayer. Imam Malik (may Allah have mercy on him) said: Between Dhat al-Nusub and Madinah there is a distance of four burud, that is, 48 miles.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5392