Bava Batra 17B

גמ׳ פתח בבור ⁠– ומסיים בכותל; ליתני: אלא אם כן הרחיק מבורו של חבירו שלשה טפחים!

Talmud: The tanna of the Mishnah opens by speaking of a pit: A person may not dig a pit close to the pit of another, and yet he ends with a reference to a wall: Unless he distances it from the wall of another. Let the tanna teach: Unless he distanced his pit or ditch 3 handbreadths from the pit of another, just as he begins his statement by referring to a pit. Why does the Mishnah suddenly mention a wall here?

אמר אביי, ואיתימא רב יהודה: ״מכותל בורו״ שנינו.

Abaye said, and some say it was Rav Yehuda who said: We learned that the Mishnah means: From the wall of his pit. In other words, one should read the Mishnah as follows: Unless he distanced his pit or ditch 3 handbreadths from the wall of another’s pit. The neighbor also built his pit close to the border between the two properties, and the Mishnah is teaching that the one digging a pit must distance it 3 handbreadths from the wall of the pit of the other.

וליתני: אלא אם כן הרחיק מבורו של חבירו שלשה טפחים! הא קא משמע לן ⁠– דכותל בור שלשה טפחים; נפקא מינה למקח וממכר ⁠– כדתניא: האומר לחבירו: ״בור וכותליה אני מוכר לך״⁠ – צריך שיהא הכותל שלשה טפחים.

The Talmud challenges: But even so, let it teach: Unless he distanced his excavations 3 handbreadths from the pit of another, and one would understand that the term pit is referring to the wall of the other’s pit. The Talmud responds: By using the phrase: From the wall of another, this teaches us incidentally that the wall of a pit must be at least 3 handbreadths thick, as the wall of the other’s pit occupied the full 3 handbreadths between the cavity of his pit and the property of his neighbor. The practical difference of this observation is with regard to buying and selling, as it is taught in a baraita: With regard to one who says to another: I am selling you a pit and its walls, the wall of the pit must be at least 3 handbreadths thick.