Rashi on Leviticus 4:18/רש"י על ויקרא ד׳:י״ח

1יסוד מזבח העלה אשר פתח אהל מועד [AND HE SHALL POUR OUT ALL THE BLOOD AT] THE BASE OF THE ALTAR OF THE BURNT-OFFERING WHICH IS AT THE ENTRANCE OF THE APPOINTED TENT — this is the base on the west side, for it was that which was opposite to the entrance of the tent (cf. Zevachim 51a).