Volume 6, Book 60, Number 284:
Narrated ‘Abdullah:
I or somebody, asked Allah’s Apostle “Which is the biggest sin in the Sight of Allah?” He said, “That
you set up a rival (in worship) to Allah though He Alone created you.” I asked, “What is next?” He
said, “Then, that you kill your son, being afraid that he may share your meals with you.” I asked,
“What is next?” He said, “That you commit illegal sexual intercourse with the wife of your neighbor.”
Then the following Verse was revealed to confirm the statement of Allah’s Apostle:
“Those who invoke not with Allah, any other god, nor kill life as Allah has forbidden except for
just cause, nor commit illegal sexual intercourse.” (25.68)