Reference for Zevachim 235:4

מהו דתימא ליבעי קדוש בכורות כמעיקרא קא משמע לן

- You might say, it requires the sanctification of the firstborn, as originally:<span class="x" onmousemove="('comment',' Though priests are not necessary, yet we require the firstborn, who officiated originally.');">⁶</span> hence it ['man'] informs us [that it is not so].

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