EXPLANATION: “No human ever filled a vessel worse than the stomach…”

On the authority of Al-Miqdaam ibn Maadiy-Karib who said: I heard the Messenger of Allah (saws) saying:
“No human ever filled a vessel worse than the stomach. Sufficient for any son of Aadam are some morsels to keep his back straight. But if it must be, then one third for his food, one third for his drink and one third for his breath.” [Ahmad, At-Tirmidhi, An-Nasa’ie, Ibn Maajah – Hadith Saheeh]

Ibn Masaweh, a Muslim doctor, said after reading this Hadith:
“If the people only used these words, they would avoid all diseases and maladies and the clinics and pharmacies would be idle.”

Another doctor, Al-Harith ibn Kalada said:
“That which has killed mankind is the introduction of food on top of food before it has been digested.”

As for spiritual benefits: humbleness of heart, strength of understanding, lessening of lower desires, lessening of personal opinions and anger, while overeating induces the opposites of all of those. Al-Hasan Al-Basri said:
“O, son of Adam, eat with one third of your stomach and drink with one third and leave one third of your stomach to breathe so that you may think.”

Ibn Umar: A man said to Ibn Umar:
“Shouldn’t I bring you some jawarish?” Ibn Umar said: “What is that?” He said: “Something which aids in digesting your food after you eat.” Ibn Umar said: “I have not eaten to being full for four months. That is not because I am not able to do so, but I was with a group of people who were hungry more than they were full.”
 
Not reaching your goals: Muhammad ibn Wasi said:
“Whoever eats little will understand and make others understand and will be clear and humble. Overeating weighs a person down and keeps him from much of what he wants [to accomplish]."

Al-Hasan Al-Basri:
“The test of Aadam (as) was food and it is your test until Qiyamah.”

And, it used to be said:
“Whoever takes control of his stomach gets control of all good deeds.”

And:
“Wisdom does not reside in a full stomach.”

One day, Al-Hasan offered some food to his companion who said: “I have eaten until I am no longer able to eat.” To which Al-Hasan said:
“SubhanAllah! Does a Muslim eat until he is no longer able to eat?”

Allah grants this world to those whom He loves and those He does not love but only grants hunger to those whom he loves.

Ash-Shafi’ee said:
“I have not filled myself in sixteen years because filling oneself makes the body heavy, removes clear understanding, induces sleep and makes one weak for worship.”

In Bukhari and Muslim:
“The believer eats with one stomach while the kaafir eats with seven stomachs.”

Meaning: The believer eats with the manners of Islam and in moderation, while the kaafir eats based on desires and gluttony and so he eats with seven stomachs.
Also:
“Food for one is enough for two and food for two is enough for three and food for three is enough for four.”

Also: From Aa’ishah:
“The family of Muhammad (saws) never filled themselves with wheat bread three days in a row from the time he came to Madinah until he passed away.”

A Muslim should not merely follow his appetites. Allah said:
“Then, they were followed by generations who neglected the prayer and followed their appetites. They will encounter a pit of fire except for those who repent…” [Maryam:59-60]

The best generations did not:
“The best generation is my generation, followed by the one after them then the one after them. Then will come a people who bear witness but are not asked to bear witness, who swear oaths but do not fulfill them and fatness will appear among them.” [Bukhari and Muslim]

From Abi Barza that the Prophet (saws) said:
“My greatest fear for you is the appetites of transgression with regard to your stomachs and your privates and the inclinations which lead astray.” [Ahmad and others and its narrators are people of Saheeh]

[By Ibn Rajab al-Hanbali - Jaami’ Al-‘Uloom wal-Hikam, Hadith No. 47]