Bava Metzia 32B

מדברי שניהם נלמד צער בעלי חיים דאורייתא. ואפילו רבי שמעון לא קאמר, אלא משום דלא מסיימי קראי. אבל מסיימי קראי דרשינן קל וחומר, משום מאי? לאו משום צער בעלי חיים דרשינן?

From the statements of both of these tanna’im it can be learned that the requirement to prevent suffering to animals is by Torah law. As even R' Shimon says that he disagreed with the opinion of the Rabbis only because the verses are not clearly defined; but had the verses been clearly defined, we would have learned the same a fortiori inference. Due to what factor can that inference be learned? What, is it not due to the matter of suffering of animals, which is a factor in unloading and not a factor in loading, that we would have learned the a fortiori inference?

דלמא משום דאיכא חסרון כיס. והכי קאמר: ומה טעינה דלית בה חסרון כיס ⁠– חייב, פריקה דאית בה חסרון כיס, לא כל שכן?!

The Talmud rejects that proof. Perhaps the a fortiori inference is due to the fact that there is the factor of monetary loss in unloading but not in loading, and this is what the Rabbis are saying: If in the case of loading, where if one fails to assist the owner there is no potential monetary loss, one is obligated to help load the animal, in the case of unloading, where if one fails to assist the owner there is potential monetary loss, is it not all the more so clear that one is required to unload the burden?

וטעינה אין בה חסרון כיס? מי לא עסקינן דאדהכי והכי בטיל משוקיה, אי נמי אתו גנבי ושקלי כל מה דאיכא בהדיה?

The Talmud asks: But is there no potential monetary loss in loading? Are we not also dealing with a case where in the meanwhile, while the owner waits for assistance, he will be prevented from bringing his merchandise to the marketplace in time to sell it; alternatively, thieves might come and take all the merchandise that is there with him? Therefore, no a fortiori inference can be learned on the basis of monetary loss, and the inference must be based on the matter of the suffering of animals.