Volume 9, Book 93, Number 623:
Narrated ‘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)