Reference for Chullin 252:7

והיכן אהלו עם אחת מהן

The covering stone and side stones of the grave convey uncleanness by contact and by overshadowing but not by carrying;<span class="x" onmousemove="('comment',' This is a traditional law and not derived from the exposition of a verse, but v. Tosaf. s.v. , and s.v. .');">⁴</span>

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