Sahih Muslim Book 4, Hadith Number 1576.

Chapter : Excellence of the forenoon prayer, two are its minimum Rak’ahs and eight are its maximum Rak’ahs and average Rak’ahs are four or six, and exhortation for the observance of the prayer.

Ibn ‘Abbas reported that the Messenger of Allah (may peace be upon him) 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…” verses 285-286 from Surah Baqara, and in the second of the two: “I believe in Allah and I bear testimony that we are Muslims” (iii. 51).