That an uncircumcised one not eat priestly tithe

The Rambam’s words

For an uncircumcised person not to partake of terumah or other consecrated foods. This concept was derived from a gezerah shavah and is not explicitly mentioned in the Torah. [Nevertheless,] based on the oral tradition, the prohibition against an uncircumcised person partaking of consecrated foods is considered as a [prohibition of] the Torah itself and not a decree of the Sages. one of the thirteen principles of Biblical exegesis in which an association is drawn between two verses based on a common word.In Sefer HaMitzvot (General Principle 2), the Rambam states that all of the 613 mitzvot are explicitly mentioned in the Torah and are not dependent on the exegesis of a verse according to the oral tradition. Therefore, he finds it necessary to explain that there is a specific oral tradition that this mitzvah is an exception to that rule.

שלא יאכל ערל תרומה, והוא הדין לשאר קודשים. .ודבר זה לימדו הכתוב מן הפסח בגזירה שווה, ואינו בפירוש מן התורה; ומפי השמועה למדו שאיסור ערל בקודשים מגופי תורה, ואינו מדברי סופרים.

Why · Sefer HaChinuch #282

That an uncircumcised one not eat priestly tithe

“It is from the roots of the commandments [that it is] because the uncircumcised is considered like a foreigner, since he did not enter the covenant of the circumcision — which is a great matter — with [the rest of] Israel.”

Who & when

“And [it] is practiced in every place that there is priestly tithe from Torah writ there, as we said in the previous commandment (Sefer HaChinukh 281).”

Where in the Mishneh Torah

Hilchot Heave Offerings הלכות תרומות