The son of Adam does not fill any vessel worse than his stomach. It is enough for the son of Adam to eat a few mouthfuls to straighten his back, but if he must (fill his stomach), then one-third for his food, one-third for his drink, and one-third for his breath

The noble Prophet (ﷺ) directs us to one of the principles of medicine, namely, to protect our health by eating little. Rather, a person should only eat the amount that maintains his survival and enables him to do the necessary activities. The worst vessel to be filled is the stomach, given the numberless ruinous diseases that result from fullness, sooner or later, and outwardly or inwardly.

Then, the Messenger (ﷺ) said: If a person must fill his stomach, let him eat as much as one-third, another one-third for drinking, and another one-third for breathing, so that he does not suffer difficulty and harm and laziness in performing the duties Allah has imposed on him concerning his religion and worldly life.