Did the Prophet (peace be upon him) say that there would be other religions after him other than the religion of Islam?
The Prophet (peace be upon him) did not say that, but he said:
There will be no prophet after me.
Allah (Exalted be He) says:
Muhammad (صلى الله عليه وسلم) is not the father of any of your men, but he is the Messenger of Allâh and the last (end) of the Prophets. And Allâh is Ever All-Aware of everything.
The Prophet (peace be upon him) warned his Ummah (nation) against what people innovate of matters that contradict his religion. The Prophet (peace be upon him) said:
He who innovates things in our affairs for which there is no valid (reason) (commits sin) and these are to be rejected.
This Hadith is agreed upon by Al-Bukhari and Muslim.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.