Imam al-Shafi'i (may Allah have mercy on him) says: When the letter of the Prophet (peace and blessings be upon him) reached Kisra, he tore it up, and he (peace and blessings be upon him) said: "His kingdom will come to an end." And we have preserved that Caesar honoured his (peace and blessings be upon him) letter and placed it in musk, and he (peace and blessings be upon him) said: "His rule will remain."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18605