Reference for Bava Metzia 189:10

But why [should be be obligated]? it is [a case of] watching with the owner: The owner of this object that was stolen was in the employ of the watchman, for also he [the guard] has a guard to him. And it is written: "If the owner is with him he shall not pay" (Shemos 22:14). And we expound later on "with him" [as] "in his employ". And even though this is written by a borrower, later we expound this also on every type of guard in Chapter "The Borrower" (Later, 95a)