Reference for Bava Metzia 64:8
(סימ"ן בהמ"ת בהמ"ת אוה"ב שונ"א רבצ"ן)
— [No] Perhaps they differ as to [the connotation of] 'under its burden,' R. Jose maintaining that we interpret 'under its burden,' a burden under which it can stand; whilst the Rabbis hold that we do not interpret 'under its burden' [thus.] [Moreover,] it may be proved that relieving the suffering of an animal is no Biblical [injunction], because the first clause states, IF HE [THE OWNER OF THE ANIMAL] WENT, SAT DOWN, AND SAID [TO THE PASSERBY], SINCE THE OBLIGATION RESTS UPON YOU TO UNLOAD, UNLOAD: HE [THE PASSER-BY] IS EXEMPT, BECAUSE IT IS SAID, 'WITH HIM'. Now, should you think that [relieving] the suffering of an animal is a Biblical injunction, what difference does it make<span class="x" onmousemove="('comment',' Lit., 'what is it to me?' ');"><sup>8</sup></span>