Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 144
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 144, with English translation by Maimonides (Rambam).
ליתן ראשית הגז לכוהן, שנאמר: "וראשית גז צאנך--תיתן לו" (דברים יח,ד).
To give the first portion of the fleece to a priest, as [Deuteronomy 18:4] states: "Give him the first portion of the shearing of your sheep."