Musannaf Ibn Abi Shayba 3:5746

Ghundar narrated to us, from Shu'bah, from Yahya ibn Yazid al-Huna'i, who said: I asked Anas ibn Malik about shortening the prayer. He said: When the Messenger of Allah (peace be upon him) would set out for a distance of three miles or three farsakhs—Shu'bah was the one who was doubtful—he would pray two rak'ahs.