Ibn ‘Umar (ra) narrated that the Prophet (saw) said: “The believer eats in one stomach whilst the kaafir eats in seven.” Narrated by al-Bukhaari (5081) and Muslim (2060).
Muhammad ibn Wasi (may Allaah have mercy upon him), 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].
Ash-Shaafi’ee (rh), said, “I have not eaten to my fill in sixteen years because filling oneself makes the body heavy, removes clear understanding, induces sleep and makes one weak for worship.”
In addition, Abu Juhayfah (ra), said, “A man burped in the presence of the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, and he said: ‘Keep your burps away from us, for the one who eats his fill the most in this world will be hungry for the longest time on the Day of Resurrection.’” [at-Tabaraani].
After hearing that, Abu Juhayfah (ra) did not eat his fill until he departed from this world; if he ate breakfast he would not eat dinner and if he ate dinner he would not eat breakfast, and he said, “I have not filled my stomach for thirty years.”
Sufyaan ath-Thawri (rh), said, “If you want your body to be healthy and to sleep less, then eat less.” Eating too much also makes the heart hard and heedless of Allaah. It was said to Imaam Ahmad (may Allaah have mercy upon him), “Does a man find any softness and humility in his heart when he is full?” He said, “I do not think so.”
In the Sahih narrated by ‘Aa’ishah (ra), “The family of Muhammad had never eaten their fill of wheat bread for three successive days since they had migrated to Madeenah till the death of the Prophet, sallallaahu ‘alayhi wa sallam”.
She (ra), also said, “The family of Muhammad (saw) did not eat two meals on one day, but one of the two was of dates.” And she (ra), said, “A complete month would pass by during which we would not make a fire (for cooking), and our food used to be only dates and water unless we were given a present of some meat.”