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.

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]‏ .‏