Rashi on Deuteronomy 22:4/רש"י על דברים כ״ב:ד׳

1הקם תקים [THOU SHALT NOT SEE THY BROTHER’S ASS OR ANY OF HIS HERD FALL DOWN BY THE WAY, AND HIDE THYSELF FROM THEM] THOU SHALT SURELY RAISE UP — This refers to the duty of loading — to re-load the burden that fell from it.

2עמו [THOU SHALT SURELY UPLOAD] WITH HIM — i.e. with the owner. But if he goes aside and sits down and says to him, “Since it is a duty for you to load it, if you want to load, go ahead and load!” — [“I” am not commanded to do it”], he is exempt from doing it (cf. Bava Metzia 32a).