It is mentioned in the Hadith: "Eat less you will be healthier."
"Nothing is worse than a person who fills his stomach. It should be enough for the son of Adam to have a few bites to satisfy his hunger. If he wishes more, it should be: one-third for his food, one-third for his liquids, and one-third for his breath." (Tarmazi, ibn Majah and Hakim)
Allah has mentioned in the Quran:
"O children of Adam! Look to your adornment at every place of worship, and eat and drink, but be not prodigals. Lo! He loveth not the prodigals." (Quran 7:31)
Now the research shows that if we eat less, its healthier and we will live longer. See the article on The Independant here.
See also my old blog on How Islam helps control Obesity.