Commentary for Numbers 35:17

באבן יד [AND IF HE SMOTE HIM] WITH A STONE IN THE HAND (more lit., with a stone of the hand) — one which has the size of a full hand (i.e., large enough to fill a hand).

Large enough. You might ask: Surely it is written, “A stone that can be held in the hand” and Rashi explains that this is “large enough to fill the hand,” meaning that it is large enough to kill. The answer is that one phrase teaches that it is large enough to kill when one strikes, and one phrase teaches that it is large enough to kill when one throws it. For sometimes it would be large enough to kill when striking but not when thrown, and if one were to throw it and kill, I might have said that he was culpable. Consequently, we are informed that he is not culpable until it is large enough to kill when thrown. With this we can answer the question of Re’m who writes “I do not know…”

אשר ימות בה signifies: which has a size sufficient to cause death, as the Targum understands it. Because it has been stated, (Exodus 21:18) “If one man smote another with a stone” but it assigned no size for it, one might think that it means, whatever size it may be! Therefore it states here, “by which one may die” (Sifrei Bamidbar 160:3).