It is Obligatory to be Silent during the Khutbah

Question :

What is the ruling on one who speaks while the Imam is delivering the sermon on Friday, when the words are not of his desire, such as when a friend greets him with salutations of peace, or there are children next to him who are talking and he tells them to be quiet?

Answer:

It is obligatory to be silent during the Khutbah on Friday, in order to listen to the Khatib and attend to the Khutbah and it is forbidden to speak at this time, even if it be ordering the good, because the Prophet said:
"If you said: "Be silent" to your companion on Friday, while the Imam is delivering the sermon, you have invalidated (your prayer).

Likewise, it is forbidden to flatten the earth or the carpet, because it has been reported in a Hadith:
"Whoever (so much as) touched a stone has invalidated (his prayer).

The Imam is exempt from this, for it is permissible for him to address those present and it is permissible for any of them to address the Imam in case of need. Other than this, it is forbidden. So if anyone greets you with salutations of peace, then reply to him with a sign. Likewise, silencing children and others should be done without speaking. If a person speaks and he is ignorant, then he is excused, but if he insists upon it even though he knows the punishment for doing so, then he is guilty of error, but he is not required to repeat the prayer. And Allah knows best.

Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 2 Pages 464-465