Musnad Ahmad 1153:2133

Muhammad ibn Bakr al-Bursani narrated to us, he said: Yahya ibn Qays informed us, he said: ‘Ata’ informed me, he said: ‘A’ishah informed me: "That the Messenger of Allah, peace and blessings of Allah be upon him, would not enter upon her after the afternoon (Asr) prayer without praying two rak'ahs with her."