Musannaf Ibn Abi Shayba 3:3966

Abu Khalid al-Ahmar narrated to us, from al-A’mash, from Ibrahim, who said: The companions of Abdullah used to recite in the two rak'ahs before the dawn (prayer) and the two rak'ahs after the sunset (prayer): "Say, 'O disbelievers'" (Surah al-Kafirun) and "Say, 'He is Allah, [who is] One'" (Surah al-Ikhlas).