(32) CHAPTER. When the Imam sees a person entering the mosque during the Khutba, he should order him to offer two Raka Salat (prayer) before sitting (Tahayyat-ul-Masjid).
Narrated Jabir bin `Abdullah: A person entered the mosque while the Prophet was delivering the Khutba on a Friday. The Prophet said to him, Have you prayed? The man replied in the negative. The Prophet said, Get up and pray two rak`at.