Urwah bin al-Zubayr (may Allah have mercy on him) narrates from Sayyidah Aishah (may Allah be pleased with her), the honoured wife of the Prophet (peace and blessings be upon him), that when Allah made the prayer obligatory, He made it obligatory as two rak'ahs. Then He completed it (to four) for the resident, and the prayer of the traveller was kept upon the original obligation.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5382