"And if stolen it shall be stolen from him": This tells me only of stealing. Whence do we derive the same for loss (of the beast)? It follows, viz.: Its being stolen involves a lapse in watching, and its being lost involves a lapse in watching. If you have learned that he is liable for the first, so, should he be liable for the second.
If Stolen It Shall Be Stolen from Him This Tells Me
Mekhilta Tractate Nezikin 16:13