Sahih Muslim Book 4, Hadith Number 1577.

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 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. 64).