The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1471
Yahya b. Yazid al-Huna'i reported: I asked Anas b. Malik about shortening of prayer. He said: When the Messenger of' Allah (may peace be upon him) had covered a distance of three miles or three farsakh (Shu'ba, one of the narrators, had some doubt about it) he observed two rak'ahs.