Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "There is no servant who conceals the faults of another servant in this worldly life, except that Allah will conceal his faults on the Day of Resurrection."
شرح الحديث :
If a Muslim finds out that His fellow Muslim is committing a sin, it is obligatory upon him to conceal it and not reveal it to the people. If he reveals it, he would be spreading immorality among the people, which is a prohibited action. Whoever conceals his brother's faults seeking Allah's reward in doing so, Allah will reward him on the Day of Resurrection by concealing his own faults and not exposing him before all those gathered on that Day.