The default is concealment: "Whoever conceals (the faults of) a Muslim, Allah will conceal his faults in this world and the Hereafter" (Muslim 2699), and in the wording of al-Bukhari 2442 and Muslim 2580, "Whoever conceals a Muslim, Allah will conceal him on the Day of Resurrection." The same ḥadīth in Muslim adds that Allah is in the aid of the servant so long as the servant is in the aid of his brother. This applies to a person who slipped and is not known for open, persistent sin — such a one is covered, advised privately, and encouraged to repent. However, concealment is not absolute: where there is ongoing harm to others — an abuser, a fraudster, or a marriage proposal from someone with a hidden danger — the scholars require that the matter be conveyed to those who can stop the harm or who need the warning, because that is naṣīḥah, not backbiting. Nor may a person expose his own sin publicly after Allah has covered it, as in the ḥadīth of the mujāhirūn: "All of my ummah will be pardoned except those who commit sins openly" (al-Bukhari 6069).
Q&A · Character
Should I conceal another Muslim's fault or expose it?
Evidence — the daleel
Sahih Muslim 2699 ṢaḥīḥSahih al-Bukhari 2442 ṢaḥīḥSahih Muslim 2580 ṢaḥīḥSahih al-Bukhari 6069 Ṣaḥīḥ
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.