Tosefta for Numbers 35:16

וְאִם־בִּכְלִ֨י בַרְזֶ֧ל ׀ הִכָּ֛הוּ וַיָּמֹ֖ת רֹצֵ֣חַֽ ה֑וּא מ֥וֹת יוּמַ֖ת הָרֹצֵֽחַ׃

But if he smote him with an instrument of iron, so that he died, he is a murderer; the murderer shall surely be put to death.