Sahih Muslim
The Book of Virtue, Good Manners and Joining of the Ties of Relationship (Kitab Al-Birr was-Salat-I-wal-Adab)
Book Number: 032 / Hadith Number: 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.