Parashas Shemos
וַתֵּ֤רֶד בַּת־פַּרְעֹה֙ לִרְחֹ֣ץ עַל־הַיְאֹ֔ר וְנַעֲרֹתֶ֥יהָ הֹלְכֹ֖ת עַל־יַ֣ד הַיְאֹ֑ר וַתֵּ֤רֶא אֶת־הַתֵּבָה֙ בְּת֣וֹךְ הַסּ֔וּף וַתִּשְׁלַ֥ח אֶת־אֲמָתָ֖הּ וַתִּקָּחֶֽהָ׃
The daughter of Pharaoh came down to bathe in the Nile, while her maidens walked along the Nile. She spied the basket among the reeds and sent her handmaid to fetch it.
Torah Temimah points to R’ Yochanan in the name of R’ Shimon ben Yochai — Sotah 12b (verified).
לרחוץ על היאור — א״ר יוחנן משום ר׳ שמעון בן יוחאי: מלמד שירדה לרחוץ מגלולי בית אביה.
English: “To bathe at the Nile” — R’ Yochanan said in the name of R’ Shimon ben Yochai: This teaches that she went down to cleanse herself of the idols of her father’s house.
Custom
Chazal teach that Bas Pharaoh’s descent to the Nile was an act of purification from her father’s idolatry.