Jami` at-Tirmidhi Book 42, Hadith 30
that Abu Sufyan bin Harb informed him that Hiraql had sent for him while he was with a party of the Quraish, and they were trading in Ash-Sham, so they went to him." And he mentioned the Hadith and said: "Then he called for the letter of the Messenger of Allah (ﷺ) to be read, and it said in it: 'In the Name of Allah, the Merciful, the Beneficent. From Muhammad, Allah's Slave and His Messenger, to Hiraql the leader of Rome. Peace be upon whoever follows the guidance. To proceed:"
"Before he died, the Messenger of Allah (ﷺ) had written to Kisra, Ceasar, An-Najashi, and to every tyrant calling them to Allah. This An-Najashi is not the one that the Prophet (ﷺ) performed to the funeral Salat for."
"When the Prophet of Allah (ﷺ) wanted to write to the foreigners, it was said to him: 'The foreigners do not accept a letter unless it has a seal.' So he had a ring made." He said: "It is as if I am now looking at its whiteness in his hand."