Tawus (may Allah have mercy on him) said: I heard Sayyiduna Ibn Abbas (may Allah be pleased with them both) saying: "The Messenger of Allah (peace and blessings be upon him) established the prayer of travel as two rak'ahs each, and established the prayer of residence as four rak'ahs."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10936