The Prophet (Sal Allahu alaihi wa sallam) said:
“He who does not show mercy to people, Allah also does not show mercy to him.” [Bukhari]
When we want to be mean to somebody or punish them, we are able to give ourselves a hundred justifications for it. This hadith gives us one huge compelling reason to put our anger and meanness aside, and practice forgiveness and gentleness instead. No one can do without Allah's mercy, so don't shut the door of goodness on yourself by being petty with others. It's not worth it!
