On page 547 (814 in the Arabic) of the earliest Muslim biography of Muhammad, Muhammad permits a threat of beheading to force a conversion to Islam:

[Muhammad] said: "Woe to you Abu Sufyan, isn't it time that you recognize that I am God's apostle?"  [Sufyan] answered, "As to that I still have some doubt."  [Ibn Abbas] said to [Sufyan], "Submit and testify that there is no God but Allah and that Muhammad is the apostle of God before you lose your head," so [Sufyan] did so.