Sahih Muslim Book 4, Hadith Number 1572.

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: I have never seen the Messenger of Allah (may peace be upon him) hastening as much in observing supererogatory as two rak’ahs before the (Fard) of the dawn prayer.