Sahih Bukhari Volume 9, Book 93, Hadith Number 611.

Narrated By ‘Abdullah : I asked Allah’s Apostle “What is the biggest sin in the sight of Allah?” He said, “To set up rivals unto Allah though He alone created you.” I said, “In fact, that is a tremendous sin,” and added, “What next?” He said, “To kill your son being afraid that he may share your food with you.” I further asked, “What next?” He said, “To commit illegal sexual intercourse with the wife of your neighbor.”