Fasting Erev Pessach - Rational and aggadic explanations.

Rambam in Hilchos Chametz Umatzah 6:12 states:


וחכמים הראשונים היו מרעיבין עצמן ערב הפסח, כדי לאכול מצה בתאווה ויהיו מצוות חביבין
עליו

The early sages hungered Erev Pessach to develop an appetite for Matzah so that Mitzvos should be pleasant.

Based on a gemara Pessachim 107-108 that Rabah did so.It would seem that is the basis for the Erev Pessach fast which then evolved into first born fasting.That minhag is mentioned in Tur based on a Massechet Soferim (Geonic source)which specifies First Borns and Tur assumes Rabah did so because he was sensitive to too much food(Istaniss). Rambam ignored that source as is his custom to stick to gemara legislation. On the occasions he deviates he is specific.

(This again confirms my earlier post about legislation nowadays not being binding unless Gemara based)

Good Shabbos