Sahih Bukhari
Prophetic Commentary on the Quran (Tafseer of the Prophet (PBUH))
Volume: 006 / Book Number: 060 / Hadith Number: 4
Narrated 'Abdullah:
I asked the Prophet, "What is the greatest sin in the Sight of Allah?" He said, "That you set up a rival unto Allah though He Alone created you." I said, "That is indeed a great sin." Then asked, "What is next?" He said, "To kill your son lest he should share your food with you." I asked, "What is next?" He said, "To commit illegal sexual intercourse with the wife of your neighbor."