Exodus 7

Moses and Aaron confront Pharaoh again. As Pharaoh scoffs, Aaron casts down his staff, which turns into a serpent, demonstrating God's power. Pharaoh's magicians duplicate the feat, but Aaron’s staff devours theirs. God then instructs Moses to unleash the first plague: Aaron strikes the Nile River, and all the water instantly turns to blood, killing the fish and leaving the Egyptians without drinking water. Pharaoh's magicians replicate this plague as well, hardening Pharaoh's heart, and he refuses to release the slaves.