Reference for Chullin 31:41

הכופה קערה על הכותל בשביל שתודח ה"ז בכי יותן הא בשביל שיודח הכותל אין זה בכי יותן

Now this is stated only in the case of a cave wall; but in the cas of a built-up wall the law is: if one placed it in order that the wall might not be damaged, the rule of 'if wat be put' does not apply; from which it follows that if one placed it in order that the wall might be washed, the rule of 'if water be put' applies.

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