Volume 8, Book 82, Number 802:
Narrated ‘Abdullah bin Mas’ud:
I said, “O Allah’s Apostle! Which is the biggest sin?” He said, “To set up rivals to Allah by worship –
ping others though He alone has created you.” I asked, “What is next?” He said, “To kill your child lest
it should share your food.” I asked, “What is next?” He said, “To commit illegal sexual intercourse
with the wife of your neighbor.”