What follows is a summary of the story concerning the events that occurred at the time of Musa عليه السلام; the victory Allah provided to His messenger and the destruction of the Pharoah. It is summarised from a khutba delivered by Shaikh Mohammad bin Salih al-Uthaymeen.
“In this month of Muharram was the salvation of Musa عليه السلام and his people from Pharoah and his army. Allah سبحانه و تعالى had sent him with clear evidences and proofs upon his prophethood, yet the Pharoah was arrogant and haughty and claimed, “I am your lord, most high” [An-Naazi’aat:24]. Musa عليه السلام came to him with great signs and called him to tawhid, to the tawhid of the Creator of the Heavens and the Earth yet Pharoah said in arrogance, “And what is the Lord of the ‘Alamin (mankind, jinns and all that exists)?”[Ash-Shu’ara:23].
Musa عليه السلام responded, “Lord of the heavens and the earth, and all that is between them, if you seek to be convinced with certainty.”[Ash-Shu’ara:24]. And so Pharoah mocked and belittled this statement of Musa عليه السلام by saying to those around, “Do you not hear (what he says)?” [Ash-Shu’ara:25]. So Musa عليه السلام reminded them and admonished them further and said, “Your Lord and the Lord of your ancient fathers!” [Ash-Shu’ara:26]. Left speechless Pharoah then replied with a false claim, a lie, “Verily, your Messenger who has been sent to you is a madman!” [Ash-Shu’ara:27]. He later continued to threaten Musa عليه السلام and said, “If you choose an ilah (god) other than me, I will certainly put you among the prisoners.” [Ash-Shu’ara:29]. Indicating that he had many other prisoners already to frighten Musa عليه السلام into silence.
Yet Musa عليه السلام continued thereafter to present the clear proofs until Pharoah said to him, “Have you come to drive us out of our land with your magic, O Musa (Moses)? “Then verily, we can produce magic the like thereof; so appoint a meeting between us and you, which neither we, nor you shall fail to keep, in an open wide place where both shall have a just and equal chance (and beholders could witness the competition).”[At-Taha: 57-58]
And when Pharoah came with his magicians Musa عليه السلام admonished them and said, “Woe unto you! Invent not a lie against Allah, lest He should destroy you completely by a torment. And surely, he who invents a lie (against Allah) will fail miserably.”[At-Taha: 61]. “Musa (Moses) said to them: “Throw what you are going to throw!” So they threw their ropes and their sticks, and said: “By the might of Fir’aun (Pharaoh), it is we who will certainly win!” Then Musa (Moses) threw his stick, and behold, it swallowed up all the falsehoods which they showed!” [Ash-Shu’ara: 43-45] And so the magicians repented and believed, “And the sorcerers fell down prostrate. Saying: “We believe in the Lord of the ‘Alamin (mankind, jinns and all that exists)” [Ash-Shu’ara: 46-47].
Pharoah continued upon his way and Allah stated, “So when they angered Us, We punished them, and drowned them all. And We made them a precedent (as a lesson for those coming after them), and an example to later generations” [Az-Zukhruf: 55-56]. Then Allah commanded Musa عليه السلام, “And indeed We inspired Musa (Moses) (saying): “Travel by night with Ibadi (My slaves) and strike a dry path for them in the sea, fearing neither to be overtaken [by Fir’aun (Pharaoh)] nor being afraid (of drowning in the sea).”[At-Taha: 77].
So Musa عليه السلام and his people exited and Pharoah was in pursuit, until they arrived at the Red Sea, Pharoah behind them and the Sea in front of them and “the companions of Musa (Moses) said: “We are sure to be overtaken.”[Ash-Shu’ara: 61]. Musa عليه السلام replied, “Nay, verily! With me is my Lord, He will guide me.”[Ash-Shu’ara: 62] That was the eemaan and certainty of Musa عليه السلام. Allah inspired Musa عليه السلام to hit the Sea with his stick and it opened into 12 pathways and so they entered and crossed with the water holding like mountains as they walked upon the dry pathways. Pharoah upon seeing that entered in chase however upon entering the mountains of water collapsed upon him, “And We took the Children of Israel across the sea, and Fir’aun (Pharaoh) with his hosts followed them in oppression and enmity, till when drowning overtook him, he said: “I believe that La ilaha illa (Huwa): (none has the right to be worshipped but) He,” in Whom the Children of Israel believe, and I am one of the Muslims (those who submit to Allah’s Will).” So it was said to him, “Now (you believe) while you refused to believe before and you were one of the Mufsidun (evil-doers, corrupts, etc.). 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 (proofs, evidences, verses, lessons, signs , revelations, etc.)” [Yunus: 90-92]
And so Allah saved Musa عليه السلامand destroyed Pharoah. It is for this reason when the Prophet صلى الله عليه و سلم entered Madinah and found the jews to be fasting on the 10th of Muharram he asked regarding that and they replied, “It is the day Allah saved Musa and his people from their enemy [Pharoah] and so Musa fasted this day”, so the prophet صلى الله عليه و سلم replied, “I am more deserving of Musa than you” and so he fasted and commanded the people to fast.
Similarly the prophet صلى الله عليه و سلم stated if he lived till the upcoming year he would fast the 9th aswell to oppose the others who were fasting only the 10th as he found them. Hence it is sunnah to fast the 9th and 10th of Muharram.
Regarding the reward of fasting the day the prophet صلى الله عليه و سلم stated, “It expiates the previous year of sins” [minor sins]. This should not be confused with the day of ‘Arafah that expiates the previous year and upcoming year.