This is what Scripture says: "and let them turn back and encamp before Pi-hahiroth" (Exodus 14:2). Here was fulfilled "and afterward they shall come out with great substance" (Genesis 15:14). The Holy One, blessed be He, said to them: This is part of the promise I made to him. Let them turn back, for all the silver and gold that Joseph had gathered was placed in Baal Zephon, and when they turned back they took it, as it is said, "the wings of a dove covered with silver" (Psalms 68:14).
The Treasure of Joseph Hidden at Baal Zephon
Yalkut Shimoni on Torah 77:14
זֶה שֶׁאָמַר הַכָּתוּב, (שמות יד, ב) "וְיָּשֻׁבוּ וְיַּחֲנוּ לִפְנֵי פִּי הַחִירוֹת" כָּאן נִתְקַיֵּם וְאַחֲרֵי כֵן יֵצְאוּ בִּרְכֻשׁ גָּדוֹל. אָמַר לָהֶם הַקָּדוֹשׁ בָּרוּךְ הוּא, זוֹ הִיא מֵהַהַבְטָחָה שֶׁהִבְטַחְתִּי לוֹ, יַחְזְרוּ לַאֲחוֹרֵיהֶם שֶׁכָּל כֶּסֶף וְזָהָב שֶׁכִּנֵּס יוֹסֵף הָיָה נָתוּן בְּבַעַל צְפוֹן, וּכְשֶׁחָזְרוּ נְטָלוּהוּ שֶׁנֶּאֱמַר, "כַּנְפֵי יוֹנָה נֶחְפָּה בַּכֶּסֶף" וְגוֹ'.