yes, it is Greatest sin !!!

Sahih Al Bukhari – Book of Prophetic Commentary On The Qur'an (Tafseer Of The Prophet (pbuh))

Volumn 006, Book 060, Hadith Number 004.
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Narated By '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."