Jami` at-Tirmidhi Book 42, Hadith 28
"The Messenger of Allah (ﷺ) ordered me to learn some statements from writings of the Jews for him, and he said: 'For indeed by Allah! I do no trust the Jews with my letters.'" He said: "Half a month did not pass before I learned it, when he (ﷺ) wanted to write to the Jews I would write it to them, and when they wrote to him I would read their letters to him."
"I entered upon the Messenger of Allah (ﷺ) while there was a scribe in front of him, and I heard him saying: 'Put the pen on your ear, for that is more conducive to the scribe remembering.'"
"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."