וְהִגַּדְתָּ֣ לְבִנְךָ֔ בַּיּ֥וֹם הַה֖וּא לֵאמֹ֑ר בַּעֲב֣וּר זֶ֗ה עָשָׂ֤ה יְהוָה֙ לִ֔י בְּצֵאתִ֖י מִמִּצְרָֽיִם׃
And thou shalt tell thy son in that day, saying: It is because of that which the LORD did for me when I came forth out of Egypt.
Read midrashic interpretation of Exodus 13:8: and classical rabbinic exegesis.