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."
USC-MSA web (English) reference: Vol. 6, Book 60, Hadith 4
Arabic reference: Book 65, Hadith 4477