Addendum: A quick Google search shows that Muslims believe that Muhammad sent a letter to the Pope. One site says the letter was received and replied to politely, nothing more. Another site says the Pope, at a mass in a cathedral, proclaimed that there was only one God and that Muhammad was a true prophet, whereupon the enraged congregation tore him to pieces.
I can’t find any record of a Pope being mauled by a mob around that time. Muslim writings claim that Emperor Heraclius was or may have been a true believer, but when the court and churchmen resisted his efforts to convert them, he backtracked and said, “I was just testing you.” So that site may have conflated (and subsequently inflated) the two stories.