Sahih Muslim Book 4, Hadith Number 1571.

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 (may peace be upon him) was not so much particular about observing supererogatory rak’ahs as in case of the two rak’ahs of the dawn prayer.