Sahih Adab al-Mufrad

It is narrated from Sayyiduna Abu Hurairah, may Allah be pleased with him, that he said: A believer is a mirror to his brother; when he sees a fault in him, he corrects it.

Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates from the Prophet (peace and blessings of Allah be upon him) that he said: "A believer is the mirror of his brother, and a believer is the brother of another believer. He safeguards his belongings, supports him in his absence, and protects him from behind."

Sayyiduna Mustawrid (may Allah be pleased with him) narrates from the Prophet (peace and blessings of Allah be upon him) that he said: "Whoever eats (i.e., obtains) something after speaking ill of a Muslim / whoever makes a Muslim a means of obtaining food. (This means that when a man has a quarrel with someone, another person goes to him and speaks ill of the one with whom he had the quarrel, and by speaking against him, he establishes his own position with the other so that he may be liked as an enemy of his enemy, and in this act of speaking ill, he is given something as a gesture of closeness.) Allah will feed him from the Hellfire in the same way, and whoever is clothed with something in return for speaking ill of a Muslim, Allah will clothe him with something from the Hellfire, and whoever stands at a place of fame and show due to speaking ill of someone (he mentions the faults of another so that he may gain fame and a position), Allah will make him stand at a place of fame and show on the Day of Resurrection. Whoever eats a morsel belonging to a Muslim, Allah will feed him a similar morsel from Hell; and whoever wears a garment belonging to a Muslim, Allah will clothe him with a garment from Hell; and whoever stands in opposition to a Muslim, Allah will stand him in opposition to Himself."