Sahih Bukhari Volume 008, Book 078, Hadith Number 626.

Narated By Abu Huraira : Allah's Apostle said, "If Khosrau is ruined, there will be no Khosrau after him; and if Caesar is ruined, there will be no Caesar after him. By Him in Whose Hand Muhammad's soul is, surely you will spend their treasures in Allah's Cause."