Is it obligatory to refute backbiting on behalf of someone who heard it but was not present in the gathering where it occurred, or is this obligation limited to those who were present? And is silent disapproval sufficient for someone who heard the backbiting but was not a party to the conversation, or is it obligatory for him to refute it verbally?

It is necessary to respond to someone who slanders his Muslim brother, due to the generality of the hadith (definition): "Whoever defends the honor of his brother, Allah will keep the Fire away from his face on the Day of Resurrection." So, whoever is present during backbiting must defend his brother's honor, or leave the situation, or condemn it in his heart and dislike the statement. Some scholars have considered silence concerning backbiting a major sin. And if you hear backbiting in your room, you must denounce it.