Parashas Masei · Passuk 35:17 · TT note 1
Chabad.org — English translation + Rashi
Torah Temimah points to Sifrei / Makkos (TT note on “אֲשֶׁר יָמוּת בָּהּ”).
English: “With which one can die” — this teaches that he is not liable unless he struck him with something capable of causing death; and elsewhere it says “and he struck him a person,” teaching that he is not liable unless he struck him on a place capable of causing death.
Halakhic conditions of liability — lethal instrument and lethal place of impact.
Capital liability for striking with a stone requires both that the stone was capable of killing and that the blow landed on a vital place.