Yes, because doing it silently defies the purpose of replying to the Salam audibly.

If a person greets you with salaam and you respond quietly and the person does not hear is this a sin?

Sins | July 3, 2016 3:14 am
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Salat al tawbah (Pray for Forgiveness, 2 Rakats Nafl). Does this have to have a particular intention for a specific sin? Or does this prayer cover all sins that has been committed? Also, Can this only be done once during the day or can we pray it as many times as we wants e.g. 8rakats (similar to the format of Tahajjud).
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I came across a person who intends to blow himself up as explained in https://islamqa.info/en/217995 I want to talk him out of it and use this Hadith about two people killing each other and both will go to Hell because the one who lived إِنَّهُ أَرَادَ قَتْلَ صَاحِبِهِ http://sunnah.com/bukhari/92/34 Can I used this Hadith to prevent him from approving suicide?