Sahih Muslim Book 4, Hadith Number 1574.

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 that the Apostle of Allah (may peace be upon him) said about the two (supererogatory) rak’ahs of the dawn: They are dearer to me than the whole world.