Imam al-Awza'i (may Allah have mercy on him) narrates that Imam al-Zuhri (may Allah have mercy on him) was asked: How was the prayer of the Prophet (peace and blessings be upon him) in Makkah before he emigrated to Madinah? He said: Urwah bin al-Zubayr (may Allah have mercy on him) informed me from Sayyidah Aishah (may Allah be pleased with her): "Allah made the prayer obligatory; at first He made it obligatory as two rak'ahs, then He completed it for those who are resident, and the prayer of travel was kept on the original obligation."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1697