For the source text click/tap here: Bava Metzia 32
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The Gemora explains this to mean that it is a mitzvah to help someone unload his animal for free, but he is not obligated to help him load for free; if he wishes, he may charge him for this. Rabbi Shimon holds that it is a mitzvah to help him load for free as well.
Rava notes: It is evident from these Tannaim that they both hold that there exists a Biblical prohibition against causing an animal distress, for they stated the following kal vachomer: If the Torah would have taught the halachah (of helping out) only by loading the animal, I would certainly have known the halachah by unloading as well, for there is suffering to the animal and there is a monetary loss to the owner.
We explore the halachic approach to animal cruelty צַעַר בַּעֲלֵי חַיִּים.