Sahih Muslim Book 4, Hadith Number 1570.

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: When it was dawn, the Messenger of Allah (may peace be upon him) observed two rak’ahs, and I would say: Does he recite only the opening chapter of the Qur’an in it?