Sahih Bukhari | Volume 8 | Hadith Number 30

Narrated 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 neighbour." 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)