Sahih Bukhari Volume 8, Book 73, Hadith Number 30.

Narated By ‘Abdullah : I said ‘O Allah’s Apostle! Which sin is the greatest?” He said, “To set up a rival unto Allah, though He Alone created you.” I said, “What next?” He said, “To kill your son lest he should share your food with you.” I further asked, “What next?” He said, “To commit illegal sexual intercourse with the wife of your neighbor.” And then Allah revealed as proof of the statement of the Prophet: ‘Those who invoke not with Allah any other god)… (to end of verse).’ (25.68)