Musnad Ahmad 1153:1244

Hisham ibn Sa'id narrated to us, Khalid narrated to us, from al-Shaybani, from 'Abd al-Rahman ibn al-Aswad, from his father, from 'Aishah, who said: "Two prayers the Prophet, peace and blessings of Allah be upon him, never abandoned, neither in secret nor in public: two rak'ahs after the 'Asr and two rak'ahs before the Fajr."