Musnad Ahmad 1153:1492

Ali narrated to us, from Khalid and Hisham, from Ibn Sirin, from Aisha: "That the Messenger of Allah, peace and blessings of Allah be upon him, used to recite in the two rak'ahs of the dawn prayer: 'Say, O disbelievers' (Al-Kafirun) and 'Say, He is Allah, [who is] One' (Al-Ikhlas)." And he narrated to us, from Khalid—meaning Ali—from Ibn Sirin, from Aisha, who said: "And the Messenger of Allah, peace and blessings of Allah be upon him, used to recite them silently."