Musnad Ahmad 27:315

Waki’ narrated to us, Isra’il narrated to us, from Abu Ishaq, from Mujahid, from Ibn ‘Umar: "That the Messenger of Allah (ﷺ) recited in the two rak'ahs before the dawn (prayer) and the two rak'ahs after the sunset (prayer), twenty-something times or ten-something times: 'Say, O disbelievers' (Surah Al-Kafirun) and 'Say, He is Allah, [who is] One' (Surah Al-Ikhlas)."