In The Name of Allaah, The Most Merciful, The Bestower of Mercy

The tyrant (ruler) of Rome wrote to Khaleefah Al-Mu’tasim intimidating him; so he commanded that the (tyrant) should be sent a reply. He then said to the scribe, ”Write, to proceed: Indeed I have read what you have written and heard your address, and the response is what you will see and not you will hear. [وَسَيَعْلَمُ الْكُفَّارُ لِمَنْ عُقْبَى الدَّارِ] ”And the disbelievers will know who gets the good end (final destination)” [Soorah Ar-Radd: Ayah: 42]

[Source: Bidaayah Wan-Nihaayah of Imaam Ibn Katheer 10/296]