Sahih Bukhari Volume 008, Book 076, Hadith Number 546.
Narated By Anas bin Malik : Allah's Prophet used to say, "A disbeliever will be brought on the Day of Resurrection and will be asked. "Suppose you had as much gold as to fill the earth, would you offer it to ransom yourself?" He will reply, "Yes." Then it will be said to him, "You were asked for something easier than that (to join none in worship with Allah (i.e. to accept Islam, but you refused)."