Abdullah bin Abbas (may Allah be pleased with them both) said that the Sunnah of the Messenger of Allah (peace and blessings be upon him) is this: "The prayer of travel is two rak'ahs, and in residence four rak'ahs. Just as there is prayer before and after the prayer of residence, so too there is prayer before and after the prayer of travel."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5506