פירוש על ביצה 30:4

From where are these words derived? – Not precisely learned from Scripture, the mixing of dishes is from the Rabbis. This is what the question is saying: What did the Rabbis cause it to rely on?

Remember – You only remember things that are liable to be forgotten.

From another that comes to make it forgotten – When Yom Tov falls on erev Shabbat, and it's close to Shabbat and will be forgotten because of Yom Tov, because you will be preparing a lot for the meal of the day, this doesn't allow for Shabbat to be honored, and the Scripture warns you to remember it (Shabbat). When you mix the mixed dishes, it turns out you will remember. You only do this for Shabbat.