From Sahih Muslim (Hadith 1692):
Ibn 'Abbas reported that the Messenger of Allah (ﷺ) used to recite in the two (supererogatory) rak'ahs of the dawn prayer:" Say: We believed in Allah and what was revealed to us" and that which is found in Surah Al-i-'lmran:" Come to that word (creed) which is common between you and us" (iii)
Reference: Muslim 1692