The Messenger of Allah ﷺ said, "When Khosrow perishes, there will be no Khosrow after him, and when Caesar perishes, there will be no Caesar after him. By the One in whose hand is the soul of Muhammad, their treasures will surely be spent in the cause of Allah."
قال رسول الله ﷺ: إذا هلك كسرى فلا كسرى بعده، وإذا هلك قيصر فلا قيصر بعده، والذي نفس محمد بيده لتنفقن كنوزهما في سبيل الله.
Bukhari narrated it in the Signs of Prophethood (3618), and Muslim in the Tribulations (2918:75), both through Zuhri, from Saeed bin Musayab, from Abu Hurairah, who mentioned it.