Parashas Shemos
וַיֵּצֵא֙ בַּיּ֣וֹם הַשֵּׁנִ֔י וְהִנֵּ֛ה שְׁנֵֽי־אֲנָשִׁ֥ים עִבְרִ֖ים נִצִּ֑ים וַיֹּ֙אמֶר֙ לָֽרָשָׁ֔ע לָ֥מָּה תַכֶּ֖ה רֵעֶֽךָ׃
When he went out the next day, he found two Hebrews fighting; so he said to the offender, “Why do you strike your fellow?”
Torah Temimah points to Reish Lakish — Sanhedrin 58b (verified).
ויאמר לרשע — אמר ריש לקיש: המגביה ידו על חבירו אע״פ שלא הכהו נקרא רשע, שנאמר ויאמר לרשע למה תכה רעך — למה הכית לא נאמר אלא למה תכה.
English: “And he said to the wicked one” — Reish Lakish said: One who raises his hand against his fellow, even though he has not [yet] struck him, is called wicked, as it is said, “Why do you strike (takeh) your fellow?” — it does not say “why did you strike,” but “why do you [intend to] strike.”
Custom
Chazal teach that raising a hand to strike is itself enough to be labeled wicked, even before the blow lands.