וְאִם־מָאֵ֥ן אַתָּ֖ה לְשַׁלֵּ֑חַ הִנֵּ֣ה אָנֹכִ֗י נֹגֵ֛ף אֶת־כָּל־גְּבוּלְךָ֖ בַּֽצְפַרְדְּעִֽים׃
And if thou refuse to let them go, behold, I will smite all thy borders with frogs.
Read kabbalistic (mystical) interpretation of Exodus 7:27: and classical mystical sources.