Rashi on Numbers 35:18/רש"י על במדבר ל״ה:י״ח

1או בכלי עץ יד OR [IF HE SMITE HIM] WITH A WEAPON OF WOOD IN HIS HAND — Because it has been stated, (Exodus 21:20) “And if a man smite his bondman or his bondwoman with a rod”, one might think that it means whatever size it may be! Therefore it states here of the wooden instrument אשר ימות בה — meaning that it must have a size sufficient to cause death (Sifrei Bamidbar 160:4).