Midrasch zu Bamidbar 35:17
וְאִ֡ם בְּאֶ֣בֶן יָד֩ אֲשֶׁר־יָמ֨וּת בָּ֥הּ הִכָּ֛הוּ וַיָּמֹ֖ת רֹצֵ֣חַֽ ה֑וּא מ֥וֹת יוּמַ֖ת הָרֹצֵֽחַ׃
Oder wenn er ihn mit einem Stein in der Hand, durch welchen man sterben kann, geschlagen hat, dass er gestorben, so ist er ein Mörder; der Mörder soll sterben.
Mekhilta d'Rabbi Yishmael
"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.
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