And We took the Children of Israel across the sea, and Fir'awn with his hosts followed them in oppression and enmity, till when drowning overtook him, he (Fira'wn) said: "I believe that none has the right to be worshipped but He in Whom the Children of Israel believe, and I am one of the Muslims. (90)Now (you believe) while you refused to believe before and you were one of the mischief-makers (91)So this day We shall deliver your (dead) body (out from the sea) that you may be a sign to those who come after you! And verily, many among mankind are heedless of Our Ayat (92)

Allah tells us how He caused Fir'awn and his soldiers to drown. The Children of Israel left Egypt in the company of Musa. It was said that there were six hundred thousand soldiers, plus offspring. They borrowed a lot of ornaments from the Coptics and took that with them. Fir'awn became very angry with them. So he sent heralds to all the cities to send their soldiers. He embarked, following behind them, filled with great pride and with massive armies. Allah wanted this to happen for He had a plan for them. No one that had any authority or power remained behind in Fir'awn's kingdom. They were all together and caught the Children of Israel at sunrise.

فَلَمَّا تَرَاءَى الْجَمْعَانِ قَالَ أَصْحَابُ مُوسَىٰ إِنَّا لَمُدْرَكُونَ

(And when the two hosts met each other, the companions of Musa said: "We are sure to be overtaken.")(26:61) They said that because when they got to the seashore Fir'awn was behind them. The two groups met face to face. The people with Musa kept asking, "How can we be saved today?" Musa replied, "I have been commanded to come this way." Musa said:

كَلَّا ۖ إِنَّ مَعِيَ رَبِّي سَيَهْدِينِ

(Nay, verily, with me is my Lord. He will guide me.)(26:62) It had been so difficult, but it suddenly became easy. Allah commanded him to strike the ocean with his staff. He did and the sea was cleft asunder, each part stood like a mighty mountain. The sea was split into twelve paths, each route for each Israelite tribe. Allah then commanded the wind and the path was dry for them.

فَاضْرِبْ لَهُمْ طَرِيقًا فِي الْبَحْرِ يَبَسًا لَّا تَخَافُ دَرَكًا وَلَا تَخْشَىٰ

(And strike a dry path for them in the sea, fearing neither to be overtaken [by Fir'awn] nor being afraid (of drowning in the sea).)(20:77)

The water in between the paths appeared as windows and every tribe was able to see the other so they would not think that others were destroyed. The Children of Israel crossed the sea. When the last one crossed, Fir'awn and his soldiers had arrived at the edge of the other shore. They were one hundred thousand black horses in addition to horsemen of other colors. When Fir'awn saw the sea he was frightened. He wanted to turn back, but it was too late. Allah's decree prevailed and the prayer of Musa was answered.

Jibril came on a war stallion. He passed by Fir'awn's horse. Jibril's horse whinnied at Fir'awn's and then Jibril rushed into the sea, and Fir'awn did the same behind him. Fir'awn no longer had any control over matters. He wanted to sound strong before his chiefs, so he said: "The Children of Israel do not have more right in the sea." So they rushed into the sea. Mika'il was behind their army pushing them all to join. When they all were in the sea and the first of them was about to emerge on the other side, Allah, the All-Powerful, commanded the sea to strand them. The sea closed over them and none was saved. The waves took them up and down. The waves accumulated above Fir'awn and he was overwhelmed by the stupors of death. While in this state, he said:

آمَنتُ أَنَّهُ لَا إِلَٰهَ إِلَّا الَّذِي آمَنَتْ بِهِ بَنُو إِسْرَائِيلَ وَأَنَا مِنَ الْمُسْلِمِينَ

(I believe that none has the right to be worshipped but He (Allah) in Whom the Children of Israel believe, and I am one of the Muslims.) He believed at a time when he couldn't benefit from his faith.

فَلَمَّا رَأَوْا بَأْسَنَا قَالُوا آمَنَّا بِاللَّهِ وَحْدَهُ وَكَفَرْنَا بِمَا كُنَّا بِهِ مُشْرِكِينَ - فَلَمْ يَكُ يَنفَعُهُمْ إِيمَانُهُمْ لَمَّا رَأَوْا بَأْسَنَا ۖ سُنَّتَ اللَّهِ الَّتِي قَدْ خَلَتْ فِي عِبَادِهِ ۖ وَخَسِرَ هُنَالِكَ الْكَافِرُونَ

(So when they saw Our punishment, they said: "We believe in Allah Alone and reject (all) that we used to associate with Him as (His) partners." Then their faith could not avail them when they saw Our punishment. (Like) this has been the way of Allah in dealing with His servants. And there the disbelievers lost utterly (when Our torment covered them).)(40:84-85) Therefore Allah said, as a response to Fir'awn,

آلْآنَ وَقَدْ عَصَيْتَ قَبْلُ

(Now (you believe) while you refused to believe before?) do you say that just now when you have disobeyed Allah before that.

وَكُنتَ مِنَ الْمُفْسِدِينَ

(And you were one of the mischief-makers.) You were among the makers of mischief on the earth who misled the people.

وَجَعَلْنَاهُمْ أَئِمَّةً يَدْعُونَ إِلَى النَّارِ ۖ وَيَوْمَ الْقِيَامَةِ لَا يُنصَرُونَ

(and We made them leaders inviting to the Fire: and on the Day of Resurrection, they will not be helped.)(28:41) These facts about Fir'awn and his status at that time were among the secrets of the Unseen that Allah revealed to His Messenger, Muhammad ﷺ. Similarly Abu Dawud At-Tayalisi recorded that Ibn 'Abbas said that Allah's Messenger ﷺ said;

قَالَ لِي جِبْرِيلُ: لَوْ رَأَيْتَنِي وَأَنَا آخِذٌ مِنْ حَالِ الْبَحْرِ فَأَدُسُّهُ فِي فَمِ فِرْعَونَ مَخَافَةَ أَنْ تُدْرِكَهُ الرَّحْمَةُ

(Jibril said to me, "If you could have seen me while I was taking black mud from the sea and placing into the mouth of Fir'awn out of fear that the mercy would reach him.") Abu 'Isa At-Tirmidhi and Ibn Jarir also recorded it. At-Tirmidhi said, "Hasan Gharib Sahih." About Allah's statement,

فَالْيَوْمَ نُنَجِّيكَ بِبَدَنِكَ لِتَكُونَ لِمَنْ خَلْفَكَ آيَةً

(So this day We shall deliver your (dead) body (out from the sea) that you may be a sign to those who come after you!) Ibn 'Abbas and others from among the Salaf have said: "Some of the Children of Israel doubted the death of Fir'awn so Allah commanded the sea to throw his body – whole, without a soul – with his known armor plate. The body was thrown to a high place on the land so that the Children of Israel could confirm his death and destruction." That is why Allah said,

فَالْيَوْمَ نُنَجِّيكَ

("So this day We shall deliver your..") meaning that We will put your body on a high place on the earth. Mujahid said,

بِبَدَنِكَ 

(your (dead) body) means, 'your physical body. "

لِتَكُونَ لِمَنْ خَلْفَكَ آيَةً

(that you may be a sign to those who come after you!) meaning, so that might be a proof of your death and destruction for the Children of Israel.

That also stood as a proof that Allah is All-Powerful, in Whose control are all the creatures. Nothing can bear His anger. Fir'awn and his people were destroyed on the day of 'Ashura', as recorded by Al-Bukhari, Ibn 'Abbas said, "When the Prophet ﷺ arrived at Al-Madinah, the Jews fasted the day of 'Ashura'. So he asked,

مَا هَذَا الْيَومُ الَّذِي تَصُومُونَهُ؟

(What is this day that you are fasting?) They responded 'This is the day in which Musa was victorious over Fir'awn.' So the Prophet ﷺ said,

أَنْتُمْ أَحَقُّ بِمُوسَى مِنْهُمْ فَصُومُوهُ

(You have more right to Musa than they, so fast it.)