Praise be to Allah.
Accusing a Muslim of doing an evil deed without proof is a major sin which deserves punishment. Talking about it to other people is a kind of gheebah (backbiting, gossip) which Allaah has forbidden and likened to a person eating the flesh of his dead brother, as He says (interpretation of the meaning):
“… neither backbite one another. Would one of you like to eat the flesh of his dead brother? You would hate it (so hate backbiting). And fear Allaah. Verily, Allaah is the One Who accepts repentance, Most Merciful.”[al-Hujuraat 49:12]
“Verily, those who like that (the crime of) illegal sexual should be propagated among those who believe, they will have a painful torment in this world and in the Hereafter…” [al-Noor 24:19].
Whoever slanders his brother or accuses him of an evil deed has to repent to Allaah, ask his brother’s forgiveness, and avoid everything that causes harm to a fellow Muslim. Abu Hurayrah (may Allaah be pleased with him) reported that the Prophet (peace and blessings of Allaah be upon him) said: “Whoever believes in Allaah and the Last Day, let him say something good or else remain silent.”
(Reported by al-Bukhaari, 11/256, and by Muslim, no. 47). Abu Dharr (may Allaah be pleased with him) said: “I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: ‘No man accuses another of an immoral deed or kufr, but it will come back on him, if the person he accuses is not as he says he is.’” (Reported by al-Bukhari, 10/388). Accusing a Muslim of drinking wine is the same as accusing him of an immoral deed (fisq), and whoever accuses his brother of something he is not guilty of, Allaah will make him fall into the thing he of which he accused his brother. A man was brought to ‘Abd-Allaah ibn ‘Abbaas, and he was told, “This is so and so, his beard was dripping with wine.” [Ibn ‘Abbaas] said, “We were forbidden to spy on others, but if something is obvious to us, we will accept it and act accordingly.” (Reported by Abu Dawood, 4890, who said it is a saheeh hasan hadeeth).
It is not permissible for the Muslim to spy on his Muslim brother or to seek out his faults. If he accidentally sees him in a doubtful place or seeming to behave in a doubtful manner, he should not accuse him unless he sees him with his own eyes doing a wrongful action. He should advise and counsel him, then if he insists on doing that thing, and there is fear that he may harm other Muslims, or if one is asked about him for a legitimate reason, then he should tell those who have something to do with it about what he knows. But he should not broadcast it unless the person does the evil action openly, because this will only help the Shaytaan against him and stop him from repenting. And Allaah knows best.