Chapter: Greeting (prayers upon entering the masjid) while the Imam is preaching
Jabir b. 'Abdullah reported that while Allah's Messenger ( صلی اللہ علیہ وسلم ) was delivering the sermon on Friday a person came there, and the Messenger of Allah ( صلی اللہ علیہ وسلم ) said to him: So and so, have you prayed (two rak'ahs)? He said: No. He (the Holy Prophet) said: Then stand and pray.
Authentic