Who said that you cant universalize your action then it is not moral?

The philosopher Immanuel Kant articulated the idea that if an action cannot be universally applied without contradiction, then it cannot be considered moral. This principle is part of his ethical framework known as the "Categorical Imperative," which emphasizes that moral actions should be based on maxims that can be consistently willed as universal laws. If an action fails this test of universality, it undermines its moral validity.