Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1549
Mu'adha asked 'A'isha (Allah be pleased with her) how many rak'ahs Allah's Messenger (may peace be upon him) prayed at the forenoon prayer. She replied: Four rak'ahs, but sometimes more as he pleased.