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: "Associating partners with Allah, especially when it is Allah Who created you." I asked: "After that, which sin is the greatest?" He said: "Committing adultery with your neighbor's wife."