The Shahadah is a testimony of faith. By saying [or believing in] this sentence, one becomes a muslim

أشهد أن لا إله إلا الله وحده لا شريك له وأشهد أن محمدا عبده ورسوله

Ašhadu an lā ilāha illā-llāh waḥdahu lā šarīka lahu, wa ašhadu anna muḥammadan ʿabduhu wa rasūluhu.

I bear witness that there is no god except Allah; One is He, He has no partner and I bear witness that Muhammad is His Servant and Messenger