Musnad Abu Ya'la al-Mawsili 150:141

'Abd al-A'la ibn Hammad narrated to us, 'Abd al-Wahhab ibn 'Abd al-Majid narrated to us, Yahya ibn Sa'id narrated to us, that 'Urwah ibn al-Zubayr narrated to him, from 'Aishah, that she said: "Indeed, the first that prayer was obligated, in travel and at home, was two rak'ahs. Then it was increased for the home, and it was kept as it was for travel."