Bava Kamma 53A

וזה וזה בבור.

and both this and that refer to where it fell into the pit itself, but nevertheless if it fell backward, he is exempt.

רב לטעמיה ⁠– דאמר רב: בור שחייבה עליו תורה ⁠– להבלו, ולא לחבטו.

The Talmud notes: Rav conforms to his line of reasoning, as Rav says: Damage classified as Pit for which the Torah obligates him to pay is referring specifically to damage caused by the pit’s lethal fumes, such as if an animal suffocates inside it, but not to damage caused by the impact of hitting the ground, for which he is exempt from paying compensation. Since the ox in this case fell backward on its back, the owner of the pit is exempt from paying compensation, as the ox wasn’t killed by the lethal fumes of the pit, but by the impact of the fall.

ושמואל אמר: בבור ⁠– בין מלפניו בין מלאחריו, חייב.

And Shmuel says: If the ox fell into the pit, whether it fell forward or whether it fell backward, he is liable.

שמואל לטעמיה, דאמר: להבלו ⁠– וכל שכן לחבטו. אלא היכי דמי לאחריו מקול הכרייה ⁠– דפטור? כגון דנתקל בבור, ונפל לאחורי הבור חוץ לבור.

The Talmud notes: Shmuel conforms to his line of reasoning, as Shmuel says: With regard to damage classified as Pit, the Torah holds one liable for damage caused by its lethal fumes, and all the more so for damage resulting from the impact of the fall. The Talmud asks: But according to Shmuel, what are the circumstances concerning which the Mishnah stated that if the ox fell backward into the pit due to the sound of the digging that one is exempt? The Talmud answers: This applies, For example: where the ox stumbled on the pit, and then fell behind the pit and was injured outside the pit.