Abu Dawud Book 2, Hadith Number 1197.

Chapter : When should a traveller shorten his prayer.

Narated By Yahya b. Yazid al-Hannani : I asked Anas b. Malik about the shortening of the prayer (while travelling). He said: The Apostle of Allah (PBUH) went out on a journey of three miles or three farakh (generally 5250 yards) (the narrator Shu’bah doubted), he used to pray two rak’ahs.