Musnad Ahmad 1153:2004

Yazid narrated to us, he said: Al-Jurayri informed us, from Abdullah ibn Shaqiq, from Aisha, she said: The Messenger of Allah (peace and blessings of Allah be upon him) used to pray four [rak'ahs] before the Dhuhr [prayer]. And Yazid said once: And two rak'ahs after it, and two rak'ahs before the Fajr [prayer]. And he used to say: "They are excellent two surahs, you recite them in the two rak'ahs before the Fajr: 'Say, O disbelievers' and 'Say, He is Allah, [who is] One'."