Hadith: 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

عن المِقْدَام بن مَعْدِي كَرِبَ رضي الله عنه قال: سمعت رسول الله صلى الله عليه وسلم يقول: «ما مَلَأ آدَمِيٌّ وِعَاءً شَرًّا من بطن، بِحَسْبِ ابن آدم أُكُلَاتٍ يُقِمْنَ صُلْبَه،ُ فإن كان لا مَحَالةَ، فَثُلُثٌ لطعامه، وثلث لشرابه، وثلث لِنَفَسِهِ».
[صحيح] - [رواه الترمذي وابن ماجه وأحمد]

Al-Miqdām ibn Ma‘di Karib (may Allah be pleased with him) reported: I heard the Messenger of Allah (may Allah's peace and blessings be upon him) say: "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."
[Authentic hadith] - [Narrated by Ibn Majah - Narrated by At-Termedhy - Narrated by Ahmad]

Explanation

The noble Prophet (may Allah's peace and blessings be upon him) guides us to one of the principles of medicine, which is the precaution that man should take to maintain his health by minimizing the amount of food he consumes. One should eat only the amount sufficient to keep him alive and provide him with strength to carry out his essential tasks. The worst vessel to fill is the stomach due to the innumerable fatal illnesses caused by satiation that appear sooner or later, whether internally or externally. Also, the Messenger of Allah (may Allah's peace and blessings be upon him) said that if one must fill his stomach, then he should let his food fill its one third only, his drink another third, and the remaining third for his breath, so that he would not suffer constriction and harm and feel too lazy to fulfill what Allah has made obligatory on him in terms of his religious and worldly affairs.