Exodus 9:6
And the LORD did that thing on the morrow, and all the cattle of Egypt died: but of the cattle of the children of Israel died not one. And the LORD did that thing on the morrow, and all the cattle of Egypt died: but of the cattle of the children of Israel died not one.
God warned Pharaoh through Moses and Aaron to let His people go to serve Him, declaring that if he refused and still held them a plague would fall on the livestock and foretelling that Israel would be exempt from the evil. The plague was murrain upon the cattle that were out in the field, brought on the morrow at the time announced beforehand. Its coming exactly as predicted, on the day foretold, showed it was not chance. All the Egyptian cattle struck by it died, including beasts of burden such as oxen, sheep, horses, camels and donkeys.
Not one of the cattle of the children of Israel died or was even afflicted, marking a complete division between the cattle of Israel and the cattle of Egypt and God's protection of Israel. That sparing of Israel's cattle while Egypt's were destroyed confirmed that the warning given through Moses had come true. Together with the other plagues it demonstrated to the Egyptians that the whole earth is under the command of the God of the Hebrews. The destruction was aimed at Egypt's worship of animals, striking sacred animals including sacred bulls, rams, heifers and goats, and by this act the Lord showed His supremacy and exposed the powerlessness of the animal idols.
Pharaoh tested the division by sending messengers to the homes of the Israelites to inquire. They reported back that Israel had entirely escaped, that not one of their cattle had died. Yet his heart was hardened and he remained obstinate in persistency, encouraged by the priests and magicians, and refused to let the people go.