Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1470
Anas b. Malik is reported to have said: I observed four rak'ahs in the noon prayer with the Messenger of Allah (may peace be upon him) at Medina, and said two rak'ahs in the afternoon prayer at Dhu'l-Hulaifa.