Aisha reported: The Messenger of Allah, peace be upon him, said:

Whoever innovates something into this matter of ours which does not belong to it will have it rejected.

Source: Sahih Bukhari 2550, Grade: Muttafaqun Alayhi

In another narration, the Prophet said:

Whoever performs a deed that is not in accordance with our matter will have it rejected.

Source: Sahih Muslim 1718, Grade: Sahih