Sahih Muslim Book 4, Hadith Number 1568.

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) used to observe two (supererogatory) rak’ahs in between the call to prayer and the Iqama of the dawn prayer.