Musnad Abu Ya'la al-Mawsili 111:26

Abu Ya'la informed us, he said: It was read to Bishr ibn al-Walid, Abu Yusuf informed you, from Abdullah ibn Ali, from Ishaq ibn Abdullah, from Talha ibn Kaysan, from Urwah, from Aisha, she said: The first thing that was obligated of the prayer was two rak'ahs. Then it was increased in the prayer of residence, and the prayer of travel was left as it was.