It is related that al-Hasan al-Basree (rahimahullaah) said:

A Muslim would be put to shame if it was said to him, "You have a big belly." [He also said]: O son of Aadam, eat for a third of your stomach, drink a third of it and leave a third of your stomach to breathe and think. [He also said]: The ordeal (calamity) of your father, Aadam (alayhis salaam) was a morsel (of food), and this will be your ordeal until the Day of Judgement.

Mawsoo'ah Ibn Abi al-Dunyaa (4/94-5) and Jaami' al-Uloom wal-Hikam (p. 553) through Hayat al-Salaf of Ahmad al-Tayyar.

Note:. That which is known from the Salaf is that they ate food in moderation and preferred periods of hunger because it softens the heart and endows wisdom and leaves the intellect and mind able to think clearly. al-Hasan al-Basree (rahimahullaah) invited a man to food and the man said, "I've already eaten and I am not able to take more", so al-Hasan said, "Subhaanallaah! Does a believer eat until he is not able to return for more?" Mawsoo'ah Ibn Abi al-Dunyaa (7/483). Note also that obesity arising due to gluttony and excess eating is what is blameworthy.

Translated by Abu Iyaad (@abuiyaadsp)