Reference for Eruvin 40:19

ת"ש בהמה שראשה ורובה בפנים אובסין אותה מבפנים והא אבוס כמאן דנקיט מנא ונקיט לה וקא בעינן ראשו ורובו מאי בהמה נמי דקתני גמל

And this may also be justified logically; for if he held the vessel only and not the cow, would [the supply of the water have been] permitted seeing that it was in fact taught: A man must not fill [a vessel with] water to hold it before his beast [on the Sabbath],<span class="x" onmousemove="('comment',' So MS.M.');">²⁸</span> but he fills [his bucket] and pours it out [into a trough] and the cow drinks of its own accord?

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