Sahih Bukhari
Punishment of Disbelievers at War with Allah and His Apostle
Volume: 008 / Book Number: 082 / Hadith 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 worshipping 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."