Za'idah bin Umayr (may Allah have mercy on him) said: I asked Sayyiduna Ibn Abbas (may Allah be pleased with them both) about the prayer while travelling, and he said: "Two rak'ahs each; this is the Sunnah of your Prophet (peace and blessings be upon him)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12610