"You shall not steal" — why is it stated? Because it says "One who steals a man and sells him" (Exodus 21:16): we have heard the punishment, but we have not heard the prohibition. Scripture teaches, saying "You shall not steal"; this then is the prohibition against one who steals a person. Go out and learn from the thirteen principles and so forth. And further, Rabbi said: Three commandments are stated in this matter, two explicit and one undefined. We learn the undefined from the explicit: just as the explicit ones are commandments for which one is liable to death by the court, so too the undefined one is a commandment for which one is liable to death by the court.
Stealing a Person as a Capital Prohibition From Context
Yalkut Shimoni on Torah 298:6
לֹא תִגְנֹב לָמָּה נֶאֱמַר, לְפִי שֶׁהוּא אוֹמֵר (שמות כא, טז) "וְגֹנֵב אִישׁ וּמְכָרוֹ" עֹנֶשׁ שָׁמַעְנוּ אַזְהָרָה לֹא שָׁמַעְנוּ, תַּלְמוּד לוֹמַר לֹא תִגְנֹב, הֲרֵי זֶה אַזְהָרָה לְגוֹנֵב נֶפֶשׁ. צֵא וּלְמַד מִשְׁלֹשׁ עֶשְׂרֵה מִדּוֹת וְכוּ', וְעוֹד אָמַר רַבִּי, שָׁלֹשׁ מִצְוֹת נֶאֶמְרוּ בְּעִנְיָן זֶה, שְׁתַּיִם מְפֹרָשׁוֹת, וְאַחַת סְתוּמָה, נִלְמַד סְתוּמָה מִמְּפֹרָשׁוֹת, מַה מְפֹרָשׁוֹת מִצְוֹת שֶׁחַיָּבִין עֲלֵיהֶן מִיתַת בֵּית דִּין, אַף סְתוּמָה מִצְוֹת שֶׁחַיָּבִין עָלֶיהָ מִיתַת בֵּית דִּין.