Arabic text
عن ابن عباس رضي الله عنهما أن النبي صلى الله عليه وسلم قال: «البَرَكة تنْزِلُ وَسَطَ الطَّعام، فَكُلُوا مِن حافَّتَيْه، ولا تأكلوا مِنْ وَسَطِه».
Translation
Ibn ‘Abbās (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Blessing descends in the middle of the food. So eat from the sides, and do not eat from the middle."
Explanation
This Hadīth highlights one of the manners of eating food when people are gathered around a bowl or a dish. The Prophet (may Allah's peace and blessings be upon him) commanded eating from the sides of the pot, without eating directly from the middle, because divine blessings come down on the middle. A blessed food, although insufficient, is palatable and satiates many people. If there is a leftover, it remains clean on the basis that no one has touched it. So those who want to eat later can do so. One, however, would be discouraged to eat the leftover if someone ate from the middle of the food, as that renders it unclean and spoils the rest of it, so it ends up being thrown away, even if it was much.
Source: HadeethEnc.com