Bismillaah:

Imam Muhammad Ibn ‘Uthaymeen رحمه الله said:

It is haraam to give salaams during the Friday khutbah, so it is not permissible for one who enters the mosque whilst the imam is delivering the khutbah to give salaams, and it is also haraam to return the greeting.

{ Al Fataawa 16/100 }

***And Allaah عزوجل knows best ***