Sahih Bukhari Volume 001, Book 002, Hadith Number 031.
Sahih Bukhari Book 02. Belief
Narated By 'Abdullah : When the following Verse was
revealed: "It is those who believe and confuse not their belief
with wrong (worshipping others besides Allah.)" (6:83), the
companions of Allah's Apostle asked, "Who is amongst us who
had not done injustice (wrong)?" Allah revealed: "No doubt,
joining others in worship with Allah is a great injustice (wrong)
indeed." (31.13)