I said, "O Allah's Messenger (ﷺ)! 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."
Reference: Sahih al-Bukhari 6811 In-book : Book 86, Hadith 41 USC-MSA web (English) : Vol. 8, Book 82, Hadith 802 (deprecated)
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