Sahih Bukhari Volume 004, Book 055, Hadith Number 551.

Narated By Anas : The Prophet said, "Allah will say to that person of the (Hell) Fire who will receive the least punishment, 'If you had everything on the earth, would you give it as a ransom to free yourself (i.e. save yourself from this Fire)?' He will say, 'Yes.' Then Allah will say, 'While you were in the backbone of Adam, I asked you much less than this, i.e. not to worship others besides Me, but you insisted on worshipping others besides me.'"