Exodus 9:1
Then the LORD said unto Moses, Go in unto Pharaoh, and tell him, Thus saith the LORD God of the Hebrews, Let my people go, that they may serve me. Then the LORD said unto Moses, Go in unto Pharaoh, and tell him, Thus saith the LORD God of the Hebrews, Let my people go, that they may serve me.
The Lord sent Moses, with Aaron, in to Pharaoh with the demand in the name of the LORD God of the Hebrews to let His people go that they might serve Him. Continued refusal to release them would bring a plague of murrain bringing death and destruction upon the cattle of Egypt. The time of the plague was stated beforehand so that it could not be said to have happened by chance.
While the cattle of Egypt died, the cattle of Israel and of the Hebrews were spared and protected so that all were preserved and not one died, making a distinction between the two peoples. This exercise of control demonstrated to the Egyptians that the whole earth is under the command of the God of the Hebrews. The judgment struck at Egyptian worship of sacred animals, exposed the impotence of their idols, and affirmed Jehovah's supremacy and Israel's deliverance.
Even with this clear sign Pharaoh hardened his heart and refused to let the people go.