Riyad as-Salihin

Ibn 'Umar (May Allah be pleased with them) reported: I observed the Prophet (ﷺ) for one month reciting in the two supererogatory Rak'ah of the Fajr prayer Surat Al-Kafirun (No. 109) [in the first Rak'ah], and Surat Al-Ikhlas (No. 112) [in the second Rak'ah].

[At-Tirmidhi].