ויושב דכשר בבמה מנלן אמר קרא (דברים י, ח) לעמוד לפני ה' לשרתו לפני ה' ולא לפני במה:
The Gemara asks: And from where do we derive that one who is sitting is fit to perform sacrificial rites on a private altar? If he is not, one can refute the common element in the same manner as above. The Gemara responds: The verse states: “The Lord separated the tribe of Levi to bear the Ark of the Covenant of the Lord, to stand before the Lord to minister to Him” (Deuteronomy 10:8). The verse indicates that the sacrificial rites must be performed while standing only before the Lord, i.e., in the Temple, where the Divine Presence resides, and not before a private altar, which is a mere place of worship.
מתני׳ כל הזבחים שקבלו דמן זר אונן טבול יום ומחוסר כפורים ומחוסר בגדים שלא רחץ ידים ורגלים ערל טמא יושב עומד על גבי כלים על גבי בהמה על גבי רגלי חברו פסל
MISHNA: With regard to all slaughtered offerings, if the one who collected their blood was a non-priest; or a priest who was an acute mourner, i.e., one whose relative has died and has not yet been buried; or one who was ritually impure who immersed that day and is waiting for nightfall for the purification process to be completed; or one who has not yet brought an atonement offering, e.g., a zav or leper after the seventh day of the purification process; or a priest lacking the requisite priestly vestments; or one who did not wash his hands and feet from the water in the Basin prior to performing the Temple service; or an uncircumcised priest; or a ritually impure priest; or if the one who collected the blood was sitting; or if he was standing not on the floor of the Temple but upon vessels, or upon an animal, or upon the feet of another, he has disqualified the offering.