Musar zu Schemot 12:45

Commandment 282 forbids an uncircumcised priest to eat תרומה. The Talmud Yevamot 70 derives this law by association with the law prohibiting any non-circumcised Jew to eat from the Passover sacrifice. Since that prohibition extends to the resident stranger and the hired hand, so does this prohibition extend to the hired hand of a priest.