Musnad Ahmad 1153:1997

Yazid narrated to us, he said: Hisham informed us, from Muhammad, that Aisha was asked about the recitation in the two rak'ahs before the dawn prayer. She said: The Messenger of Allah (peace and blessings of Allah be upon him) used to recite silently in them, and she mentioned: "Say, 'O disbelievers'" (Surah Al-Kafirun) and "Say, 'He is Allah, [who is] One'" (Surah Al-Ikhlas).