The statement that is NOT one of the Five Pillars of Islam is:
"Muslims must truly believe that there is no God but Muhammad."
The correct belief is that "there is no god but Allah, and Muhammad is his messenger." The other statements are part of the Five Pillars of Islam:
- Muslims must pray five times a day (Salah).
- Muslims must be generous to the poor (Zakat).
- Muslims must fast during Ramadan (Sawm).
So, the incorrect statement pertains to the core tenet of faith.