I was reading Rav Melamed's Peninei Halacha, the Shabbat volume, and on page 5 he mentions that when the Jews were slaves in Egypt they had scrolls (megillot) in their possession telling of the forthcoming redemption that they would read on shabbat.
The source is from Shemot Rabbah 5:18:
מְלַמֵּד שֶׁהָיוּ בְיָדָם מְגִלּוֹת שֶׁהָיוּ מִשְׁתַּעְשְׁעִין בָּהֶם מִשַּׁבָּת לְשַׁבָּת, לוֹמַר שֶׁהַקָּדוֹשׁ בָּרוּךְ הוּא גּוֹאֲלָן, לְפִי שֶׁהָיוּ נָחִין בְּשַׁבָּת, אָמַר לָהֶן פַּרְעֹה, תִּכְבַּד הָעֲבוֹדָה עַל הָאֲנָשִׁים וְיַעֲשׂוּ בָהּ וְאַל יִשְׁעוּ וגו', אַל יְהוּ מִשְׁתַּעַשְׁעִין וְאַל יְהוּ נְפִישִׁין בְּיוֹם הַשַּׁבָּת.
Is there any more information about who would have written these scrolls, what they contained and who transmitted them through the years in Egypt? Is there any other mention of this in scripture?