Musannaf Ibn Abi Shayba 3:3959

Abu Bakr narrated to us, he said: Abu al-Ahwas narrated to us, from Abu Ishaq, from Mujahid, from Ibn Umar, who said: I heard the Prophet, may Allah bless him and grant him peace, more than twenty times reciting in the two rak'ahs before the dawn [prayer] and the two rak'ahs after the sunset [prayer]: "Say, 'O disbelievers'" and "Say, 'He is Allah, [who is] One.'"