Narrated `Abdullah (radiAllahu anhu): I asked the Prophet (sallellahu alaihi wa sallam):
“What is the greatest sin in the Sight of Allah?”
He said, “That you set up a rival unto Allah though He Alone created you.”
I said, “That is indeed a great sin.” Then asked, “What is next?”
He said, “To kill your son lest he should share your food with you.”
I asked, “What is next?”
He said, “To commit illegal sexual intercourse with the wife of your neighbour.”
[Bukhari: 6: 5]