Sahih Bukhari Volume 9, Book 93, Hadith Number 623.

Narrated By ‘Abdullah : A man said, “O Allah’s Apostle! Which sin is the biggest in Allah’s Sight?” The Prophet said, “To set up rivals unto Allah though He Alone created you.” That man said, “What is next?” The Prophet said, “To kill your son lest he should share your food with you.” The man said, “What is next?” The Prophet said, “To commit illegal sexual intercourse with the wife of your neighbor.” Then Allah revealed in confirmation of that: “And those who invoke not with Allah any other god, nor kill such life as Allah has made sacred except for just cause, nor commit illegal sexual intercourse and whoever does this shall receive the punishment… (25.68)