ANSWER:

He salla Allahu alaihi wa sallam would dictate it to others to write for him and he would put a seal on the letters with his finger ring as his signature. 

sheikh.we know that Muhammad (pbuh) couldn’t write so how he wrote a letter to Hiraclious?

Biography of the Prophet salla Allahu alahi wa sallam | February 10, 2019 4:13 am
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Sheikh what is the difference between allah sending sallah upon prophet muhammed (peace be upon him) and the ummah sending sallah upon him (peace be upon him)
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With the Prophet ﷺ‎ Episode 20 (How did the Prophet ﷺ‎ deal with Women? 2 ) – Sheikh Assim Al Hakeem