Where in code validate/sanitize inputs?

What is the best practice to put validation/sanitization logic? In controllers or rather in models? Since from what i understand, controllers operate on users demands and accepts input from them i guess that would be for me logical place to process data there, and keep models for interworking, when data is safe already, but then, i can use same model function for sanitizing data in multiple places. I am kinda new to MVC, and since i am self-taught I am missing so some basic concepts :X

I searched forum but closest answer was posted years ago.

Thread about my project that is using CodeIgniter:

Topic: Mysticat.pet - online game themed around cats