The khateeb instructing the worshippers to send blessings upon the Prophet (blessings and peace of Allah be upon him) during the khutbah

Praise be to Allah.

This is not prescribed according to Islamic teachings, because if those who have come to listen to the khutbah send blessings upon the Prophet (blessings and peace of Allah be upon him), they should do so quietly, and not say it out loud.

Shaykh Ibn Baaz (may Allah have mercy on him) was asked: During the Jumu’ah khutbah, some imams say: Let us hear you send blessings upon the Prophet from time to time, and more than five or six times during a single khutbah. And he says that in colloquial Arabic. Is that permissible?

He replied:

That is not prescribed, that is not prescribed; he should not say, Let us hear you. Rather what the people must do is listen attentively during the khutbah. What is required of the congregation is to listen attentively to the khateeb so that they can benefit from his khutbah, and if a person quietly sends blessings upon the Prophet (blessings and peace of Allah be upon him) when he hears him being mentioned, this is Sunnah. But he should not raise his voice; rather he should say it quietly to himself so as not to disturb anyone else. End quote.

http://www.binbaz.org.sa/mat/16424.

Shaykh Saalih al-Fawzaan (may Allah preserve him) quoted Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) as saying:

Raising one’s voice in front of the khateeb is either disliked (makrooh) or prohibited according to scholarly consensus, and neither the mu’adhdhin nor anyone else should raise his voice in sending blessings upon the Prophet (blessings and peace of Allah be upon him) or any other words. End quote.

He commented further on that by saying: it should be noted that what the shaykh referred to is still practised in some countries, namely raising the voice in sending blessings upon the Prophet (blessings and peace of Allah be upon him) or other du‘aa’s during the khutbah, or before it, or between the two khutbahs, and some khateebs may even instruct those who are present to do that. This is an act of ignorance and innovation and it is not permissible to do that." (Al-Mulakhkhas al-Fiqhi p. 120, 121).

And Allah knows best.