Answered by Shaykh Faraz Rabbani
Question: Is it sunnah not to finish all the food on your plate, and to leave some food for next time? I ask because someone told me this is a very good thing to do, and someone else said this is illogical. Please clarify this, insha’Allah.
Leaving food is from wastefulness: leaving just a little is better not to; leaving more than this is blameworthy; that which would be considered ‘excessively wasteful’ is sinful.
This is if the food on one’s plate got there of one’s own accord. Otherwise, the waste is from the one who gave too much.