Sahih Bukhari: Hadith 5786

I said 'O Allah's Messenger (ﷺ)! 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)