הלכה על ביצה 4:10

And nonetheless it is specifically [regarding] things that are the same for all people's bodies that we permit and we say that it is included in this dispensation of food for the soul - such as washing the feet, as everyone washes like this sometimes. But that which is not the same for every person, such as mugmar (a type of incense) - since not all people burn mugmar, as we say in Ketuvot 7a - is certainly forbidden [along with] all that is similar to it. And it is only in these things that we need that it be the same for every person. However regarding eating, every person is permitted to make even a food that is not its way to be made except by kings and great ministers - since the basic concept of eating is something that is the same for every soul. And so [too,] that which they said (Beitzah 2b) about the prohibition of preparation, that a holiday cannot prepare for Shabbat, nor a Shabbat prepare for a holiday. And you will only find a prohibition of preparation from Torah writ in my opinion - if you have studied the words of the Gemara well - with an egg alone. And its main prohibition is when it was born on a holiday after Shabbat or on a Shabbat after a holiday. In this way is the preparation of Rabbah established with an egg, and with this is there a Torah prohibition - and in no other way. As behold, that which we prohibit when it is born on Shabbat on the holiday that is after it, and so [too,] when it is born on the holiday on the Shabbat that is after it, and so [too,] when it it is born on the holiday [on the holiday] itself is [only] a rabbinic prohibition - and it is all on account of a decree [to make a fence for] the one that is born on a holiday after Shabbat, as we said. And the matter would be [too] lengthy if I came to write it with a broad elucidation. And in its place at the beginning of Beitzah, I have written at length about it, as my teachers - may God protect them - have taught me.