Sahih Bukhari
Oaths and Vows
Volume: 008 / Book Number: 078 / Hadith Number: 626
Narrated 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."