Musnad Ahmad 1153:1009

'Affan narrated to us, he said: Shu'bah narrated to us, from Abu Ishaq, he said: I heard al-Aswad ibn Yazid and Masruq saying: We bear witness regarding 'Aishah that she said: "The Messenger of Allah (peace and blessings of Allah be upon him) was never with me on a day but that he prayed two rak'ahs after the 'Asr prayer."