And they journeyed from Elim. That very day was the Sabbath in which they complained, as it is stated, "And the people murmured against Moses" (Exodus 16:2). And Moses assured them that they would receive food the next day, as it is written, "And in the morning, you shall see the glory of the Lord" (Exodus 16:7). On the first day of the week, which was a Sabbath, the manna descended for them from the beginning, as it is written, "On the sixth day, they shall prepare what they bring in" (Exodus 16:5).
Moses and Creation of Sabbath
Yalkut Shimoni on Torah 787:1
(במדבר לג י) וַיִּסְעוּ מֵאֵילִם. אוֹתוֹ הַיּוֹם שַׁבָּת הָיָה, שֶׁבּוֹ בַּיּוֹם נִתְרָעֲמוּ, שֶׁנֶּאֱמַר (שמות טז, ב) "וַיִּלּוֹנוּ (הָעָם) [כָל עֲדַת בְנֵי יִשְׂרָאֵל] עַל מֹשֶׁה", וְהִבְטִיחָן לְהוֹרִיד לָהֶם מָן לְמָחֳרַת, דִּכְתִיב (שם, ז) "וּבֹקֶר וּרְאִיתֶם אֶת כְּבוֹד ה'". בְּאֶחָד בְּשַׁבָּת יָרַד לָהֶם מָן תְּחִלָּה, דִּכְתִיב "שֵׁשֶׁת יָמִים תִּלְקְטֻ(וּ)הוּ".