Midrash for Deuteronomy 18:7
וְשֵׁרֵ֕ת בְּשֵׁ֖ם יְהוָ֣ה אֱלֹהָ֑יו כְּכָל־אֶחָיו֙ הַלְוִיִּ֔ם הָעֹמְדִ֥ים שָׁ֖ם לִפְנֵ֥י יְהוָֽה׃
then he shall minister in the name of the LORD his God, as all his brethren the Levites do, who stand there before the LORD.
Sifrei Devarim
(Devarim 18:7) "And he shall serve in the name of the L-rd his G-d, as all his brothers, the Levites, who stand there before the L-rd": He stands upon the floor. From here they ruled: If he sat or stood upon vessels or upon a beast or upon the feet of his fellow, (his service) is invalid.
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