From Sahih Muslim (Hadith 1691):
Ibn 'Abbas reported that the Messenger of Allah (ﷺ) used to recite in first of the two (supererogatory) rak'ahs of the dawn:"Say: We believed in Allah and what was revealed to us..." verse 136 from Surah Baqara, and in the second of the two: "I believe in Allah and I bear testimony that we are Muslims" (iii)
Reference: Muslim 1691