Reference for Zevachim 41:1

והא פרה דטמויי מטמינן ליה דתנן מטמאין היו הכהן השורף את הפרה ומטבילין אותו להוציא מלבן של צדוקין שהיו אומרים במעורבי שמש היתה נעשית ש"מ לא פסלה בה (מחשבת) טומאה

Now in the case of the [red] heifer we defile him, for we learnt: They used to defile the priest who was to burn the heifer and then make him immerse, in order to combat the opinion of the Sadducees,<span class="x" onmousemove="('comment',' V. Sanh. (Sonc.ed.) p. 353. n. 2.');">¹</span> who maintained: It[s service] was performed [only] by [priests] who had experienced sunset!<span class="x" onmousemove="('comment',' l.e., by priests upon whom the sun had set after their immersion, as in the case of the sacrificial service in general. The Rabbis however held that immediately after immersion (when he is called a tebul yom v. Glos) a priest was fit for the burning of the red heifer. V. Parah III, 7.');">²</span>

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