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

عَنْ المِقْدَامِ بْنِ مَعْدِي كَرِبَ رَضِيَ اللَّهُ عَنْهُ قَالَ: سَمِعْتُ رَسُولَ اللهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ:
«مَا مَلأَ آدَمِيٌّ وِعَاءً شَرًّا مِنْ بَطْنٍ، بِحَسْبِ ابْنِ آدَمَ أَكَلاَتٌ يُقِمْنَ صُلْبَهُ، فَإِنْ كَانَ لَا مَحَالَةَ، فَثُلُثٌ لِطَعَامِهِ، وَثُلُثٌ لِشَرَابِهِ، وَثُلُثٌ لِنَفَسِهِ».
[صحيح] - [رواه الإمام أحمد والترمذي والنسائي وابن ماجه] - [الأربعون النووية: 47]

Al-Miqdām ibn Ma‘di Karib (may Allah be pleased with him) reported: I heard the Messenger of Allah (ﷺ) 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] - [Narrated by Imam Ahmad, At-Tirmidhi, An-Nasā’i, and Ibn Mājah] - [Al-Arba‘oon An-Nawawiyyah (The Forty Hadith of An-Nawawiyyah) - 47]

Explanation

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.

Benefits from the Hadith

1. We should not eat and drink much. This is a principle that combines all principles of medicine, given the numerous illnesses and maladies that result from satiety.
2. The purpose behind eating is to maintain one's health and strength, which are necessary for the soundness of life.
3. Filling the stomach with food results in physical and religious harmful effects. ‘Umar (may Allah be pleased with him) said: "Beware of gluttony, for it harms the body and causes laziness toward the prayer."
4. Eating, in terms of its ruling, falls into various categories: Obligatory: that which preserves life, and neglecting it would lead to harm. Permissible: what goes beyond the obligatory amount without fear of harm. Disliked: what carries a risk of harm. Prohibited: what is known to cause harm. Recommended: what aids a person in worshiping and obeying Allah. This is summarized in the Hadīth in three levels: 1. Filling the stomach. 2. A few bites or morsels that keep the back straight. 3. His saying: "One-third for his food, one-third for his drink, and one-third for his breath." And all of this applies when the type of food itself is lawful.
5. The Hadīth is a principle from among the principles of medicine. Since the science of medicine revolves around three foundations—preservation of strength, dietary regulation, and elimination—the Hadīth encompasses the first two of them, as in the statement of Allah Almighty: {Eat and drink, but do not waste, for He does not like the wasteful.} [Surat al-A‘rāf: 31]
6. The Hadīth demonstrates the perfection of this Shariah, as it includes people's interests concerning their religion and worldly life.
7. The sciences of Shariah include the principles of medicine and various types of it, like what is reported on honey and the black seed.
8. The rulings of the Shariah contain wisdom and are based on the principle of warding off evils and bringing about benefits.