Tafsir of Al-Mujadilah 58:8

The Jews and the hypocrites used to whisper among themselves and wink at one another whenever they saw the believers, intending to enrage them. The Messenger of Allah (ﷺ) forbade them, but they returned to their behavior. Their whispering consisted of sin, aggression against the believers, and mutual encouragement to disobey and oppose the Messenger. It is also recited as yantajūn (whispering) with a kasra under the 'ayn.

Regarding their disobedience to the Messenger, {they greet you with that which Allah does not greet you}, it means they say in their greeting to you: "As-sām (death) be upon you, O Muhammad." As-sām means death, while Allah the Exalted says: {And peace be upon His servants whom He has chosen} (An-Naml: 59).

Regarding {Why does Allah not punish us for what we say?}, they used to say: "If he were truly a prophet, why does he not pray against us so that Allah might punish us for what we say?" Allah the Exalted replied: {Hell is sufficient for them} as a punishment.

{O you who have believed, when you converse privately, do not converse about sin and aggression and disobedience to the Messenger; but converse about righteousness and piety. And fear Allah, to whom you will be gathered. Private conversation is only from Satan that he may grieve those who have believed, but he will not harm them at all except by permission of Allah. And upon Allah let the believers rely.}