Musnad Ahmad 32:373

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Yahya ibn Yazid al-Huna’i, who said: I asked Anas ibn Malik about shortening the prayer. He said: I used to go out to Kufa and pray two rak‘ahs until I returned. And Anas said: "When the Messenger of Allah (peace and blessings of Allah be upon him) would go out for a distance of three miles, or three farsakhs — Shu‘bah is the one who was in doubt — he would pray two rak‘ahs."