שמות ט:ז פירוש: חזקוני

וישלח פרעה והנה לא מת, “Pharaoh dispatched people to check on the livestock of the Israelites, and not a single one had died.” The reason for this was Moses’ prediction in verse 4 that G-d will make this distinction between Egyptian and Israelite owned beasts.

ויכבד לבו; “but his heart remained obstinate;” he did not ask Moses to pray, as the dead beasts could not be brought back to life. The beasts that had survived did not need to be prayed for.