וַאֲנִ֣י יָדַ֔עְתִּי כִּ֠י לֹֽא־יִתֵּ֥ן אֶתְכֶ֛ם מֶ֥לֶךְ מִצְרַ֖יִם לַהֲלֹ֑ךְ וְלֹ֖א בְּיָ֥ד חֲזָקָֽה׃
And I know that the king of Egypt will not give you leave to go, except by a mighty hand.
Read kabbalistic (mystical) interpretation of Exodus 3:19: and classical mystical sources.