From Sahih Muslim (Hadith 1583):
Yahya b. Yazid al-Huna'i reported:I asked Anas b. Malik about shortening of prayer. He said: When the Messenger of' Allah (ﷺ) 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
Reference: Muslim 1583