Respect your fellow Muslims and do not talk unjustly against their honour.

This hadith, narrated by AbuHurayrah, teaches us that it is a major sin to insult or abuse another Muslim. Going to lengths in talking unjustly against a Muslim's honour is the gravest sin of all. This means that we should not say anything bad about our fellow Muslims and should instead be kind and respectful towards them. We should also remember that even if someone has wronged us, it does not give us the right to insult them in return as this would be considered a major sin according to this hadith.