The Law of Constant proportion states

Joseph Proust (1754-1826) formulated the law of definite proportions (also called the Law of Constant Composition or Proust's Law). This law states that if a compound is broken down into its constituent elements, the masses of the constituents will always have the same proportions, regardless of the quantity or source of the original substance. 

This fundamental principle, also known as the Law of Constant Composition or Proust’s Law, asserts that the elements that make up a particular compound are always present in a fixed, constant ratio by mass. Regardless of the sample's origin, whether it is large or small, or from different sources, the proportions of the elements in the compound will remain the same. This discovery was crucial in advancing the understanding of chemical reactions and compounds, laying the groundwork for the concept that chemical compounds have unique and consistent compositions.