《礼记·曲礼上》
这是害怕弄脏了国君的食器。吃剩余之食不须行祭食之礼。父亲吃儿子剩余之食，丈夫吃妻子剩余之食，也都不祭。
御同于长者，虽贰不辞，偶坐不辞。
This is out of fear of contaminating the sovereign's food vessel. When consuming leftover food, it is not necessary to perform the offering food ritual. When a father consumes his son's leftover food, and the husband consumes his wife's leftover food, there is also no ritual offering.