Reference for Zevachim 230:9

והכל קרבו עולות ועולה שהקריבו ישראל במדבר טעונה הפשט וניתוח ונכרים בזמן הזה רשאין לעשות כן

For it was taught: And let the priests also, that come near to the Lord, sanctify themselves:<span class="x" onmousemove="('comment',' Ex. XIX, 22. This was immediately before Revelation, while the incident cited above took place immediately after Revelation.');">¹⁰</span> R'Joshua B'Karhah said: This intimated the separation of the first born.<span class="x" onmousemove="('comment',' By 'priests' the firstborn are meant here, as it was they who 'came near the Lord' to perform sacrifices, and the verse now separated them and forbade them to approach the mountain.');">¹¹</span>

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