Sahih Muslim Book 4, Hadith Number 1573.

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.

‘Aisha reported Allah’s Messenger as saying: The two rak’ahs at dawn are better than this world and what it contains.