Sahih Bukhari

Narrated `Abdullah:I asked the Prophet, ""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 neighbor