Exodus 9:7
And Pharaoh sent, and, behold, there was not one of the cattle of the Israelites dead. And the heart of Pharaoh was hardened, and he did not let the people go. And Pharaoh sent, and, behold, there was not one of the cattle of the Israelites dead. And the heart of Pharaoh was hardened, and he did not let the people go.
Pharaoh was told beforehand that a deadly murrain would fall upon all the Egyptian cattle in the field, while it was distinctly declared that the Hebrews would be exempt, and the plague was then sent exactly as predicted. Its appointed time was announced before it came, so that it could not be dismissed as happening by chance. The murrain struck and destroyed the Egyptian cattle in the field.
The blow fell directly upon Egypt's system of animal worship, destroying even their sacred animals. In contrast, not one of the cattle of the Israelites died, confirming the stated distinction between Israel and Egypt. Indeed none had died or even been afflicted by the plague, but had entirely escaped unharmed. The difference displayed Jehovah's supremacy and the impotence of Egypt's animal gods.
Pharaoh sent messengers to the homes of the Israelites to verify the report, and they found and reported that Israel had been spared. Despite this clear evidence his heart was hardened and he remained obstinate, refusing to let the people go. He was encouraged in his persistency by the priests and magicians. The preservation was meant to demonstrate to the Egyptians that the whole earth is under the command of the God of the Hebrews.