Sahih Muslim Book 4, Hadith Number 1566.

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.

Hafsa reported: When the dawn appeared, the Apostle of Allah (may peace be upon him) observed two rak’ahs (of Sunnah prayers).