Rashi on Exodus 4:2/רש"י על שמות ד׳:ב׳

1מזה בידך WHAT IS THAT IN THINE HAND? —The reason why the words מה זה are written here as one word is to afford the opportunity to explain it thus: on account of this (מִזֶּה) that is in thine hand thou wilt make thyself liable to punishment because thou hast harboured suspicion about worthy persons (Exodus Rabbah 3:12). The real meaning, however, is that it is a question such as one puts to his fellow: “You admit that this object in front of you is a stone?” The other replies, “Yes”. The former says, “Well, I’ll turn it into wood”.