Sahih Bukhari Volume 008, Book 082, Hadith Number 802.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 82. Punishment of Disbelievers at War with Allah and his Apostle
Narrated By '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."