Musnad Abu Ya'la al-Mawsili 150:436

Abu Bakr narrated to us, Ali bin Mushir narrated to us, from al-Shaybani, from Abd al-Rahman bin al-Aswad, from his father, from Aisha, who said: "Two prayers the Messenger of Allah (peace and blessings of Allah be upon him) never abandoned, neither in secret nor in public: two rak'ahs before the dawn (Fajr) and two rak'ahs after the afternoon (Asr)."