The food for two persons is sufficient for three, and the food of three persons is sufficient for four persons."

Bismilahirrahmanirrahim

✦ Miqdam bin Madikarib Radi Allahu anhu said I heard the Messenger of Allah (ﷺ) say: A human being fills no worse vessel than his stomach. It is sufficient for a human being to eat a few mouthfuls to keep his spine straight. But if he must (fill it), then one third of food, one third for drink and one third for air.

Sunan Ibn majah, Book 29, 3349-Sahih

✦ Narrated Abu Huraira Radi AllahuAnhu: ALLAH's Apostle Sal-Allahu Alaihi Wasallam said, "The food for two persons is sufficient for three, and the food of three persons is sufficient for four persons." 

Sahih Bukhari, Vol 7, Book 65, No. 304