Musnad Ahmad 1153:1949

Abu Ahmad Muhammad ibn Abdullah ibn al-Zubayr narrated to us, he said: Usamah ibn Zayd al-Laythi narrated to us, from al-Qasim ibn Muhammad, from Aisha, she said: "The prayer was ordained as two rak'ahs. Then the Messenger of Allah, peace and blessings of Allah be upon him, added to the prayer of residence, and left the prayer of travel as it was."