This mitzvah is not explicit in the text. Rather, it is derived by means of a gezeira shava, a tradition of similar laws applied in cases with identical phraseology. The Talmud in Yevamos (70a) equates the case of the korban Pesach in Exodus 12:45 (Mitzvah #14) with the case of terumah in Leviticus 22:10. Both use the words toshav and sachir to restrict certain people from eating the item in question. The Talmud infers that just as the former also restricts an arel (uncircumcised man), so does the latter. (This is still a Biblical, rather than a rabbinic, prohibition.)
There are legitimate reasons for not being circumcised – hemophilia, for one. Nevertheless, if a kohein isn’t circumcised, even for a good reason, he may not eat terumah. The reason is that an uncircumcised person is in some ways considered an “outsider” (AKA a “zar,” or a non-kohein). As we discussed, terumah is “members only” for kohanim and his uncircumcised status renders his membership to more of an “associate member” status.
This mitzvah applies to kohanim when the laws of terumah are in effect. It is discussed in the Talmud in tractate Yevamos on page 70a. It is codified in the Mishneh Torah in the seventh chapter of Hilchos Terumos. This mitzvah is #135 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos.