Proust's Law of Chemical proportion states that-

The French chemist Joseph Proust stated this law the following way: "A chemical compound always contains the same elements combined together in the same proportion by mass." This implies that any pure sample of a compound, no matter the source, will always consist of the same elements that are present in the same ratio by mass. For example, pure water will always contain hydrogen and oxygen in a fixed mass ratio (a gram of water consists of approximately 0.11 grams of hydrogen and 0.88 grams of oxygen, the ratio is 1:8).