Rashi on Exodus 22:13/רש"י על שמות כ״ב:י״ג

1וכי ישאל איש AND IF A MAN BORROWS — The purpose of this verse is to tell you that the borrower is liable for loss from whatever cause even from accident.

2בעליו אין עמו THE OWNER THEREOF NOT BEING WITH IT (lit., with “him”) — i. e. if the owner of the ox is not employed with the borrower in his work (Bava Metzia 95b).