It is obligatory (Wajib) to remain completely silent during the Khutbah, and it is narrated in the Hadith that if someone is speaking, then speaking to make him quiet is also impermissible. Therefore, when the Imam recites the noble verse 'Innal-laha wa mala'ikatahu yusalluna 'alan-Nabiy...' etc., the followers (Muqtadis) should recite Durood Shareef in their hearts; reciting it with the tongue is not correct. When performing prayer during the Khutbah is impermissible, then reciting Durood Shareef is impermissible to a greater degree (Badarja-e-Awla). (Aham Masail / Vol: 1 / Pg: 87)