כִּי־יִמָּצֵ֣א אִ֗ישׁ גֹּנֵ֨ב נֶ֤פֶשׁ מֵאֶחָיו֙ מִבְּנֵ֣י יִשְׂרָאֵ֔ל וְהִתְעַמֶּר־בּ֖וֹ וּמְכָר֑וֹ וּמֵת֙ הַגַּנָּ֣ב הַה֔וּא וּבִֽעַרְתָּ֥ הָרָ֖ע מִקִּרְבֶּֽךָ׃
Wenn ein Mann gefunden wird, der einen seiner Brüder der Kinder Israel stiehlt, und er als Sklave mit ihm umgeht und ihn verkauft; dann wird dieser Dieb sterben; so sollst du das Böse aus deiner Mitte vertreiben.
Rashi on Deuteronomy
כי ימצא IF [A MAN] BE FOUND [STEALING …] — Found when doing so by witnesses and after legal warning. This, too, is the meaning in all cases where a form of the verb מצא is used in the Torah, under similar circumstances (Mekhilta d'Rabbi Yishmael 21:16:1; Sifrei Devarim 273:1).
Tur HaArokh
גונב גפש מאחיו, “kidnapping a person from among his brethren;” The reason why the Torah added the words מבני ישראל, is that Esau, and his descendants are also “brethren,” and would have been included in this legislation unless specifically excluded.
Siftei Chakhamim
Through witnesses, and after a warning, etc. Rashi says this as you might think that since the Torah [seemingly] makes this law dependent on the [victim] being found in the perpetrator’s hand [then this law applies] even if there were no witnesses when he stole him. Therefore Rashi tells us that generally, “to be found” generally means with witnesses. Otherwise, the perpetrator could claim that he did not steal him but that he sold himself.