BT San 43a(8) :היכא דמחייה באבן אחת ולא מית הם בגוה נהדר ונקטליה‏ : if he struck him with a stone (in the execution) and he did not die, he should execute him a second time also by means of it [i.e. the same stone]
See the entire entry for hm adv. also