וַיֹּ֣אמֶר לְאַבְרָ֗ם יָדֹ֨עַ תֵּדַ֜ע כִּי־גֵ֣ר ׀ יִהְיֶ֣ה זַרְעֲךָ֗ בְּאֶ֙רֶץ֙ לֹ֣א לָהֶ֔ם וַעֲבָד֖וּם וְעִנּ֣וּ אֹתָ֑ם אַרְבַּ֥ע מֵא֖וֹת שָׁנָֽה׃
And He said unto Abram: ‘Know of a surety that thy seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them four hundred years;
Read kabbalistic (mystical) interpretation of Genesis 15:13: and classical mystical sources.