Sahih Muslim Book 4, Hadith Number 1543.

Chapter : Excellence of greeting the mosque with two Rak’ahs and undesirability of sitting down before observing them.

Jabir b. ‘Abdullah reported: The Messenger of Allah (may peace be upon him) bought a camel from me. When he came back to Medina, he ordered me to come to the mosque and observed two rak’ahs of prayer.