It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that once I asked the Prophet (peace and blessings be upon him): "Which sin is the greatest?" The Prophet (peace and blessings be upon him) said: "To associate partners with Allah, especially when He alone has created you; to kill your child out of fear that he will eat with you; and to commit adultery with your neighbor's wife."