Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 135

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.

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.