Musnad Ahmad 25:1409

Marwan ibn Mu'awiyah narrated to us, Humayd ibn Ali al-'Uqayli informed us, al-Dahhak ibn Muzahim narrated to us, from Ibn Abbas, who said: "The Messenger of Allah (peace be upon him) prayed two rak'ahs when he traveled, and four when he stayed." Ibn Abbas said: "So whoever prays four in travel is like one who prays two in residence." He said: And Ibn Abbas said: "He did not shorten the prayer except one time, when the Messenger of Allah (peace be upon him) prayed two rak'ahs, and the people prayed one rak'ah and one rak'ah."