It is forbidden to speak during the khutbah

The majority of the scholars are of the opinion that it is obligatory
to be silent during the khutbah, and one is not to indulge in
conversation during the khutbah, not even if it is to order one to do
some good or to stop some evil, and this rule applies whether or not
the person sitting in the mosque can actually hear the khutbah.

Ibn 'Abbas reports that the Prophet sallallahu alehi wasallam said:
"Whoever speaks in Jumu'ah while the imam is delivering the khutbah
is like a donkey who is carrying books, and for those who tell him to
be quiet, there is no [reward] for the Jumu'ah."
This is related by Ahmad, ibn abi-Shaibah, al-Bazzar, and at-
Tabarani. Ibn Hajar said in Bulugh alMaram: "There is no fault in its
chain."

'Abdullah ibn 'Amr reports that the Messenger of Allah (sallallahu
alehi wasallam) said:
"There are three types of people who attend the Jumu'ah: one, a man
who is present but speaks [during the speech], and that is his
portion of the prayer; two, a man who is present and makes
supplications - in his case, Allah may give him what he asks, if He
wishes, or He may not give him what he asks, three, a person who is
present and observes silence and does not step over the necks of the
Muslims nor harm anyone - for him, there is expiation from that
Jumu'ah until the next Jumu'ah plus an additional three days as Allah
has said: 'He that does good shall have ten times as much to his
credit.'"
This is related by Ahmad and Abu Dawud with a good chain.

Abu Hurairah reports that the Prophet(sallallahu alehi wasallam)
said:
"If, during the Jumu'ah while the imam is delivering khutbah, you
tell your companion to be quiet, then you have spoken needlessly."
This is related by the group, save Ibn Majah.

Abu ad-Darda' says: "The Prophet was upon the pulpit and was
addressing the people and he recited a verse, and next to me was
Ubayy ibn-Ka'b and I asked him: When was that verse revealed?' He
refused to talk to me until the Messenger of Allah(sallallahu alehi
wasallam) came down from the pulpit and then he said to me: 'You have
nothing from your Jumu'ah, except your useless talk.' When the Prophet
(sallallahu alehi wasallam) had finished, I went to him and informed
him of what had happened, and he said:
'Ubayy has told the truth. If you hear your imam speaking, be quiet
until he is finished.''' This is related by Ahmad and at-Tabarani.

Ahmad and ash-Shaf'i are reported to have made a distinction,
concerning this ruling, between one who can hear the speech and the
one who cannot hear the speech, saying that speaking is forbidden for
the former and not for the latter, although it is preferred for the
latter also to be silent.

At-Tirmidhi records that Ahmad and Ishaq made an exception for
replying to a salutation and responding to a sneeze while the imam is
delivering the Friday khutbah. According to ash-Shaf'i:
"If a person sneezes [during the khutbah] and someone says: 'May
Allah bless you,' I wish I could have accomadated it since such a
reply is a sunnah. In my view it is makruh that a person should greet
someone with salam [while they are listening to the khutbah]. [What
makes it worse is] that his salam is not returned, even though saying
salam is a sunnah while responding to it is a fard.
(Volume 2, Number 142 - Fiqh-us-Sunnah)



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"I put my trust in Allah, my Lord and your Lord! There is not a moving creature, but He has a grasp of its forelock. Verily, my Lord is on the straight path. (The truth)"
(11:55-56)