מדרש על בבא מציעא 63:22

"on the way": and not in the stall — whence it was ruled: If it were found (fallen) in the stall, the obligation does not obtain; if in the public thoroughfare, it does.

"lift up shall you lift up": If he righted it, and it fell; righted it, and it fell — even four or five times — he must continue righting it, it being written "lift up shall you lift up." If the owner left (his fallen animal), sat down, and said to him: Since you have a mitzvah to unload, unload, he is exempt from doing so, it being written with him" (the owner). I might think that this were so even if he (the owner) were old or afflicted with boils; it is, therefore, written "lift up shall you lift up" (i.e., even in the latter instance).