Musnad Ahmad 1153:1481

'Abd al-Wahhab al-Thaqafi narrated to us, from Ayyub, from Muhammad, that 'Aishah was asked about the two rak'ahs of the dawn [prayer]. She said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to lighten them." She said: "And I think he used to recite in them something like 'Say, O disbelievers' (Qul ya ayyuha al-kafirun) and 'Say, He is Allah, [who is] One' (Qul huwallahu ahad)."