Sahih Muslim Book 4, Hadith Number 1302.

Chapter : Desirability of observing the noon prayer at the earlier hour (of times prescribed for it) when there is no intense heat.

Abu Umama b. Sahl reported: We offered the noon prayer with Umar b. ‘Abd al-‘Aziz. We then set out till we came to Anas b. Malik and found him busy in saying the afternoon prayer. I said to him: O uncle! which is this prayer that you are offering? He said: It is the afternoon prayer and this is the prayer of the Messenger of Allah (may peace be upon him) that we offered along with him.