What did the Prophet Muhammad (peace be upon him) say is the greatest sin in Allah's sight?
 Narrated `Abdullah:                     A man said, "O Allah's Apostle! Which sin is the greatest in Allah's Sight?" The Prophet said, "To set up a rival unto Allah though He Alone created you . " The man said, "What is next?" The Prophet said, "To kill your son lest he should share your food with you." The man said, "What is next?" The Prophet said, "To commit illegal sexual intercourse with the wife of your neighbor." So Allah revealed in confirmation of this narration:-- 'And those who invoke not with Allah, any other god. Nor kill, such life as Allah has forbidden except for just cause nor commit illegal sexual intercourse. And whoever does this shall receive the punishment.' (25.68)