All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The saying that the honorable questioner has asked about is not a Hadeeth, but is a saying of men of medicine. The first one to say it was Al-Haarith ibn Kildah who also said that eating a lot without leaving a chance for digestion is the main cause of death.
Therefore the Prophet, sallallaahu ‘alayhi wa sallam, whose guidance is the best to be followed, guided us to eat small amounts of food to the extent that he stated that a few mouthfuls are sufficient for the son of Adam to lead a healthy life, but if a person’s desire for food overwhelms him or her, the person should assign a third of his stomach for food, a third for drink, and leave a third empty for breath. [Abu Daawood, At-Tirmithi, An-Nasaa’i and Al-Haakim: Saheeh] Ibn Rajab Al-Hanbali pointed out that this Hadeeth encompasses all medical principles.
It was reported that when Ibn Maasawayh read this Hadeeth, he said that had these words been applied by people, they would not have been diseased and doctors and pharmacists would have had nothing to do, since the source of all diseases is gluttony.
And Allaah Knows best.