Salim bin Abdullah (may Allah have mercy on him) narrates from his father, Abdullah bin Umar (may Allah be pleased with them both), that he set out for the place Rim and shortened the prayer over that distance. Imam Malik (may Allah have mercy on him) said: Its distance too was equal to four burud, that is, 48 miles.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5393