بُلُوغُ الْمَرَامِ
Bulugh al-Maram 433
The Book of Prayer
وَعَنْ أَنَسٍ رَضِيَ اَللَّهُ عَنْهُ قَالَ: { كَانَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -إِذَا خَرَجَ مَسِيرَةَ ثَلَاثَةِ أَمْيَال ٍ [1] أَوْ فَرَاسِخَ, صَلَّى رَكْعَتَيْنِ } رَوَاهُ مُسْلِمٌ . [2] .
Narrated Anas (RA): When Allah's Messenger (ﷺ) went out on a journey of three miles or three Farasikh, he (ﷺ) used to pray two Rak'at. [Reported by Muslim]. Farasikh (plural of Farsakh) is Persian unit of distance. Farsakh is equal to about three miles.
Sunnah.com reference: Book 2, Hadith 343