Parashas Vaeira
וַיְדַבֵּ֣ר יְהֹוָה֮ אֶל־מֹשֶׁ֣ה וְאֶֽל־אַהֲרֹן֒ וַיְצַוֵּם֙ אֶל־בְּנֵ֣י יִשְׂרָאֵ֔ל וְאֶל־פַּרְעֹ֖ה מֶ֣לֶךְ מִצְרָ֑יִם לְהוֹצִ֥יא אֶת־בְּנֵֽי־יִשְׂרָאֵ֖ל מֵאֶ֥רֶץ מִצְרָֽיִם׃ {ס}
So GOD spoke to both Moses and Aaron in regard to the Israelites and Pharaoh king of Egypt, instructing them to deliver the Israelites from the land of Egypt.
Torah Temimah points to R’ Shmuel bar Rav Yitzchak — Yerushalmi / TT (verified).
ויצום אל בני ישראל — א״ר שמואל בר רב יצחק: על מה צום – על פרשת שלוח עבדים.
English: “And He commanded them regarding the children of Israel” — R’ Shmuel bar Rav Yitzchak said: Concerning what did He command them? Concerning the portion of sending forth slaves.
Custom
Chazal connect this command to the future mitzvah of releasing Hebrew slaves.