Sahih Bukhari Volume 4, Book 54, Number 445
Narrated Abu Dhar:
The Prophet said, "Gabriel said to me, 'Whoever amongst your followers die without having worshipped others besides Allah, will enter Paradise (or will not enter the (Hell) Fire)." The Prophet asked. "Even if he has committed illegal sexual intercourse or theft?" He replied, "Even then."

This is even confirmed in Quran:


4:48 "Allah forgiveth not that partners should be set up with Him; but He forgiveth anything else, to whom He pleaseth; to set up partners with Allah is to devise a sin Most heinous indeed."