The Book of Virtue, Good Manners and Joining of the Ties of Relationship (Kitab Al-Birr was-Salat-I-wa'l-Adab)
Muslim :: Book 32 : Hadith 6267
Abu Huraira reported Allah's Apostle (may peace be upon him) as saying: The servant (who conceals) the faults of others in this world, Allah would conceal his faults on the Day of Resurrection.