Short answer
The Qur'an says that God (﷾) saved Pharaoh's body when he drowned, so that it would be a sign for later people. Pharaoh chased the Children of Israel (the Israelites) into the sea. As the water closed over him, he said he believed in the God (﷾) of Israel. It was too late. God (﷾) told him his body would be saved that day as a sign. The Bible does not say this. Exodus says the sea covered Pharaoh's army, and that Israel saw the Egyptians dead on the shore. It gives no last words of Pharaoh, and nothing about his body being a sign. So the verse was not copied from the Bible. Muslims hold that the Prophet Muhammad (ﷺ) received it from God (﷾).
Full explanation
What the Qur'an says
The Qur'an tells how Pharaoh followed the Children of Israel (the Israelites) into the sea. As he drowned, he declared that there is no god but the God (﷾) of Israel. He was told that the time for that had passed. His body would be saved that day, to be a sign for those who came after him. This is the Qur'an that the Prophet (ﷺ) recited in seventh-century Arabia.
“We took the Children of Israel across the sea: Pharaoh and his hosts followed them in insolence and spite. At length, when overwhelmed with the flood, he said: "I believe that there is no god except Him Whom the Children of Israel believe in: I am of those who submit (to Allah in Islam)." (It was said to him): "Ah now!- But a little while before, wast thou in rebellion!- and thou didst mischief (and violence)! "This day shall We save thee in the body, that thou mayest be a sign to those who come after thee! but verily, many among mankind are heedless of Our Signs!"”
What the Bible says
Exodus says the waters covered Pharaoh's chariots, horsemen and army, and that not one was left. It adds that Israel saw the Egyptians dead on the shore. A psalm (a sacred song) says God (﷾) overthrew Pharaoh and his army in the Red Sea. Exodus speaks of the Egyptians in general, seen dead by Israel that day. It does not single out Pharaoh's own body. It does not give his confession. It does not say his body would be a sign to later generations.
“And the waters returned, and covered the chariots, and the horsemen, and all the host of Pharaoh that came into the sea after them; there remained not so much as one of them.”
“Thus the Lord saved Israel that day out of the hand of the Egyptians; and Israel saw the Egyptians dead upon the sea shore.”
“But overthrew Pharaoh and his host in the Red sea: for his mercy endureth for ever.”
Why this was not copied
An old charge against the Qur'an is that the Prophet (ﷺ) took its stories from the Bible. But a copyist writes what his source says, mistakes included. Here the Qur'an gives two things the Bible does not have. One is Pharaoh's confession of faith as he died. The other is God's (﷾) promise about his body. A man working from the Bible could not have found either one there.
“We took the Children of Israel across the sea: Pharaoh and his hosts followed them in insolence and spite. At length, when overwhelmed with the flood, he said: "I believe that there is no god except Him Whom the Children of Israel believe in: I am of those who submit (to Allah in Islam)." (It was said to him): "Ah now!- But a little while before, wast thou in rebellion!- and thou didst mischief (and violence)! "This day shall We save thee in the body, that thou mayest be a sign to those who come after thee! but verily, many among mankind are heedless of Our Signs!"”
“And the waters returned, and covered the chariots, and the horsemen, and all the host of Pharaoh that came into the sea after them; there remained not so much as one of them.”
The same pattern elsewhere
The same thing happens in other places. The Bible calls the ruler of Egypt in Joseph's day "Pharaoh". The Qur'an calls him "the king", and keeps "Pharaoh" for the ruler Moses faced. Why that is a correction is explained on the page about the king and Pharaoh. Exodus says Aaron, the brother of Moses, made the golden calf (an idol the Israelites worshipped). The Qur'an says Aaron warned the people against it. In both cases the Qur'an departs from the Bible, to correct it or to clear a prophet. With Pharaoh's body it adds what the Bible does not have. Copying does not work this way.
“The king (of Egypt) said: "I do see (in a vision) seven fat kine, whom seven lean ones devour, and seven green ears of corn, and seven (others) withered. O ye chiefs! Expound to me my vision if it be that ye can interpret visions."”
“Then after them We sent Moses with Our signs to Pharaoh and his chiefs, but they wrongfully rejected them: So see what was the end of those who made mischief.”
“And it came to pass at the end of two full years, that Pharaoh dreamed: and, behold, he stood by the river.”
“Aaron had already, before this said to them: "O my people! ye are being tested in this: for verily your Lord is (Allah) Most Gracious; so follow me and obey my command."”
“And he received them at their hand, and fashioned it with a graving tool, after he had made it a molten calf: and they said, These be thy gods, O Israel, which brought thee up out of the land of Egypt.”
“We took the Children of Israel across the sea: Pharaoh and his hosts followed them in insolence and spite. At length, when overwhelmed with the flood, he said: "I believe that there is no god except Him Whom the Children of Israel believe in: I am of those who submit (to Allah in Islam)." (It was said to him): "Ah now!- But a little while before, wast thou in rebellion!- and thou didst mischief (and violence)! "This day shall We save thee in the body, that thou mayest be a sign to those who come after thee! but verily, many among mankind are heedless of Our Signs!"”
Perspectives
The Islamic perspective
The Qur'an is revealed by God (﷾) to the Prophet Muhammad (ﷺ). Its account of Pharaoh has things the Bible does not have, which shows it was not copied from the Bible.
The Qur'an says that the drowning Pharaoh declared faith in the God (﷾) of Israel. It says God (﷾) then saved his body that day as a sign for those who came after him.
The Bible says the sea covered Pharaoh's army and Israel saw the Egyptians dead on the shore. It gives no last words of Pharaoh, and says nothing of God (﷾) saving his body as a sign. Exodus speaks of the Egyptians in general, seen dead by Israel that day. It does not single out Pharaoh's own body.
The Bible has no confession by the drowning Pharaoh and no promise that his body would be a sign to later generations. So these words cannot have been copied from it.
The Qur'an's departures from the Bible follow a pattern. It corrects the Bible's history, clears the prophets of the sins the Bible charges them with, and adds what the Bible lacks.
The Qur'an is the word of God (﷾), not a retelling of earlier books.