Assalaamu alaykum wa rahmatullaahi wa barakaatuhu. Should I still forbid evil if the evil thing already ended? For example, the cigarette was completely used up, so the smoker stopped his current smoking session, or the music concert is over and the people are leaving, or the gang has stopped beating the innocent girl. Should I still advise them about their sins or try to make them stop doing it, or should I leave them alone since they have stopped doing the evil deed.
All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
If you see a person committing an evil, then you are obliged to forbid him from doing it. If he stops committing that evil in a natural way, such as when the cigarette that he was smoking has been extinguished and so on, then this does not prevent advising him and clarifying to him that smoking cigarettes is forbidden and that he must repent to Allah from it and not repeat that action.
The same applies to someone who listens to music, insults others, or beats someone unjustly and then stopped doing that because of achieving his purpose and not because of repenting to Allah; you have to clarify to him, advise him, and inform him that what he did is forbidden and that he must stop doing so out of obedience and repentance to Allah.
Allah knows best.
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