Midrasz do Kapłańska 7:36

2) (Vayikra 7:36) ("which the L–rd commanded to give to them on the day He anointed them, from the children of Israel, a statute forever, throughout their generations.") "which the L–rd commanded to give to them": R. Shimon said: Whence is it derived that Israel separated the gifts of the priesthood (the breast and the shok of their peace-offerings [and gave them to the first-born]) from Mount Sinai (viz. Shemoth 24:5) and that they did not revert to Aaron and his sons until they were anointed with the oil of anointment? From "which the L–rd commanded to give to them on the day He anointed them." "from the children of Israel": (only) by the consent of (all of) Israel. "a statute forever": for the eternal house (the Temple). "throughout their generations": (The original oil of anointment "holds") for all the generations (even in the absence of the oil of anointment).