Musannaf Ibn Abi Shayba 34:260

Muhammad ibn Mus’ab narrated to us, from al-Awza’i, from al-Zuhri, from ‘Urwah, from ‘Aishah, who said: Allah ordained the prayer, when He first ordained it, as two rak’ahs. Then He completed it for the resident, and the prayer of travel was kept upon the first obligation.