Sahih Muslim Book 4, Hadith Number 1562.

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.

Ibn ‘Umar reported that Hafsa, the Mother of the Believers, informed him that when the Mu’adhdhin became silent after calling (people) to the dawn prayer, the Messenger of Allah (may peace be upon him) commenced the dawn (prayer) when it dawned by observing two short rak’ahs before the commencement of the (Fard) prayer.