וַיִּשְׁלַ֣ח פַּרְעֹ֔ה וְהִנֵּ֗ה לֹא־מֵ֛ת מִמִּקְנֵ֥ה יִשְׂרָאֵ֖ל עַד־אֶחָ֑ד וַיִּכְבַּד֙ לֵ֣ב פַּרְעֹ֔ה וְלֹ֥א שִׁלַּ֖ח אֶת־הָעָֽם׃ (פ)
And Pharaoh sent, and, behold, there was not so much as one of the cattle of the Israelites dead. But the heart of Pharaoh was stubborn, and he did not let the people go.
Read kabbalistic (mystical) interpretation of Exodus 9:7: and classical mystical sources.