Musnad Abu Ya'la al-Mawsili 147:17

Abu Bakr ibn Abi Shaybah narrated to us, Muhammad ibn Ja'far narrated to us, from Shu'bah, from Yahya ibn Yazid al-Hana'i, who said: I asked Anas ibn Malik about shortening the prayer. He said: "When the Messenger of Allah (peace and blessings of Allah 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."