Humayd al-Damri (may Allah have mercy on him) said: I asked Sayyiduna Ibn Abbas (may Allah be pleased with them both): "I travel, so do I shorten the prayer while travelling?" Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: "It is not a shortening; rather this is its complete form and the Sunnah of the Prophet (peace and blessings be upon him). The Messenger of Allah (peace and blessings be upon him) went out in a state of safety, fearing none but Allah, and yet he prayed two (rak'ahs) until he returned."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11221