It was taught: "And if a man strike his fellow with a stone or with his fist" (Exodus 21:18). Shimon the Timnite says: just as a fist is distinctive in that it can be observed by witnesses, and so on, this comes to exclude the case where a stone flew from beneath the hands of the witnesses [a stone whose throwing the witnesses did not actually watch]. Rabbi Akiva said to him: and did he strike him in the presence of witnesses, that they should know with what he struck him and where he struck him, whether he struck him on his thigh or on the bird of his soul [a vital spot]?
Stone or Fist and the Witnesses Who See the Blow
Yalkut Shimoni on Torah 331:2
תַּנְיָא, וְהִכָּה אִישׁ אֶת רֵעֵהוּ בְּאֶבֶן אוֹ בְאֶגְרֹף, שִׁמְעוֹן הַתִּימְנִי אוֹמֵר, מָה אֶגְרֹף מְיֻחָד שֶׁמָּסוּר לְעֵדִים וְכוּ', פְּרָט לְשֶׁיָּצְתָה אֶבֶן מִתַּחַת יָדָן שֶׁל עֵדִים. אֲמַר לֵיהּ רַבִּי עֲקִיבָא, וְכִי בִּפְנֵי עֵדִים הִכָּהוּ, שֶׁיּוֹדְעִין בַּמֶה הִכָּהוּ וְעַל מָה הִכָּהוּ אִם עַל שׁוֹקוֹ אוֹ עַל צִפֹּר נַפְשׁוֹ הִכָּהוּ.