What mechanic would be used for taking an opponents weapon from it’s sheath on his belt in combat?


Here is the scenario.

I am attacking a fighter with a crossbow. I run out of bolts and am unnamed as he charges at me with a spear. My opponent has a short sword on his belt. He misses with his initial spear attack and its my turn.

I want to take his sword from it’s sheath and if possible attack him with it. What process or series of checks need to be made for this to work?