Is there a commentary on Mishneh Torah that connects the Rambam's words to his source in the gemara? I'm looking for something similar to the Be'er Hagola to the Shulhan Arukh, or an inverse En Mishpat Ner Misvah.
- The Rashash wrote a comment called מקורי הרמב"ם here
- In the Rambam Fraenckel edition you can find all mekorot here.
- In the Rambam Hashalem moreh shiur of Chabad there is a fine work with sources of each statememt in rambam here
- An other very interesting book is the book of Rav Menachem Casher about the midrashe Halacha used by the Rambam here
Between the מגיד משנה and מגדל עז, both printed in virtually all modern prints of the משנה תורה, at least one shows the source for most statements.