Musnad Ahmad 1153:331

Waki' narrated to us, he said: Shu'bah narrated to us, from Ibrahim ibn Muhammad ibn al-Muntashir, from his father, who said: I heard Aisha saying: "The Messenger of Allah, may Allah bless him and grant him peace, would not abandon four [rak'ahs] before the Dhuhr [prayer], and two rak'ahs before the Fajr [prayer], under any circumstances."