Sahih Muslim Book 4, Hadith Number 1900.

Chapter : Shortness of prayer and surmon.

Jabir b. ‘Abdullah reported that a person came (in the Mosque) while the Apostle of Allah (may peace be upon him) was delivering the sermon on Friday (standing) on the pulpit. He (the Holy Prophet) said to him: Have you offered two rak’ahs? He said: No. Upon this he said: Then observe (them).