Musnad Ahmad 1153:2319

Ya'qub narrated to us, he said: My father narrated to us, from Ibn Ishaq, he said: Salih ibn Kaysan narrated to me, from 'Urwah ibn al-Zubayr, from 'Aishah, the wife of the Prophet, may Allah bless him and grant him peace, she said: "The first thing that was made obligatory upon the Messenger of Allah, may Allah bless him and grant him peace, regarding prayer was two rak'ahs, two rak'ahs, except for Maghrib, for it was three. Then Allah completed the Dhuhr, the 'Asr, and the final 'Isha' to four [rak'ahs] while at home (in residence), and He kept the prayer upon its initial obligation during travel."