Reference for Bava Kamma 63:18

א"ר יוחנן הלכה כאיסי בן יהודה ומי אמר רבי יוחנן הכי והאמר ר' יוחנן הלכה כסתם משנה ותנן אחד רץ ואחד מהלך או שהיו שניהם רצין פטורין

WHERE THE CARRIER OF THE BEAM WAS IN FRONT AND THE CARRIER OF THE BARREL BEHIND, AND THE BARREL BROKE BY [COLLISION WITH] THE BEAM, HE IS EXEMPT. BUT IF THE CARRIER OF THE BEAM [SUDDENLY] STOPPED HE IS LIABLE. For surely [this latter case] here is similar to that of the lying cow kicking the walking cow,<span class="x" onmousemove="('comment',' As here, too, the offender is to blame for misconduct. ');">¹⁵</span>

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