Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.142.
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 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 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 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 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-Tirmizhi 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 accommodated 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.