Sahih Muslim Book 4, Hadith Number 1569.

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 Messenger of Allah (may peace be upon him) observed two rak’ahs of the dawn prayer and he shortened them (to the extent) that I (out of surprise) said: Did he recite in them Surah Fatiha (only)?