מדרש על במדבר 35:17

"with stone or fist": I might think that he is liable (only if he strikes) with these, but otherwise he is not liable; it is, therefore, written (Numbers 35:17) "And if with a hand-stone (i.e., a stone the size of a full hand) (whereby he can die he strike him"). He is not liable until he strikes him with something that has the potential to kill and in a locus which is critical to life.