Hadith Qudsi (Sacred Hadith) : Shirk (polytheism) is a grievous sin
From Authentic sources Hadith Qudsi (Sacred Hadith)
6) Narrated Abu Huraira 
: Allah`s Messenger 
said: Among all partners, I am the most dispensable with association. So anyone who performs a deed in which he ascribes to Me others; I will abandon him and his act of polytheism.
(This Hadith is sound and reported by Muslim).
Shirk (polytheism) is a sin that if its doer dies without repentance, he will never be forgiven. Verily, whoever sets up partners in worship with Allah, Allah has forbidden Paradise for him; and the Fire will be his abode.
