Sahih Bukhari Volume 004, Book 053, Hadith Number 349.

Narated By Abu Huraira : Allah's Apostle said, "When Khosrau is ruined, there will be no Khosrau after him; and when Caesar is ruined, there will be no Caesar after him. By Him in Whose Hands my life is, you will spend their treasures in Allah's Cause."