How does a disbeliever or an apostate embrace Islam if he is mute or has a speech impediment?

The repentance of a disbeliever or an apostate is by uttering the two testimonies of faith (Shahadatayn). This is a condition for entering Islam for one who is able to pronounce them. However, if someone is unable to pronounce them, such as a lisping person or a mute, then that does not harm them. The Islam of a mute person is accepted through writing or through a sign indicating their acceptance of Islam.