Musnad Ahmad 1153:1129

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Ibrahim ibn Muhammad ibn al-Muntashir, who narrated that he heard his father narrating that he heard 'Aishah say: "The Messenger of Allah, peace and blessings of Allah be upon him, would not abandon four [rak'ahs] before the Dhuhr [prayer], and two rak'ahs before the morning [prayer]."