Surah Yunus, Ayat 91-92

Ayat 91. Now (Firaun (Pharaoh) you believe) while you refused to believe before and you were one of the Mufsidun (evil-doers, corrupts, etc.).

Ayat 92. So this day We shall deliver your (Firaun's (Pharaoh) 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.). 

Hazrat Ibn Abbas (Allah be pleased with both of them) reported that when Messenger of Allah  [SAWW](PBUH) came to Medina, he found the Jews observing the fast on the day of Ashura. They (the Jews) were asked about it and they said: It is the day on which Allah granted victory to Moses and (his people) Bani Israel over the Pharaoh and we observe fast out of gratitude to Him. Upon this the Messenger of Allah  [SAWW](PBUH)  said: We have a closer connection with Moses than you have, and he commanded to observe fast on this day. [Muslim Book 6, Number 2518]

Lesson : as mentioned above in Surah Yunus Ayat 92. "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.). In this Hadith it is stated that when the Prophet (PBUH) emigrated from Makkah to Al-Madinah, he saw that the Jews were fasting on this day. He asked them why they did this on that day, and they told him that they did it for the reason that Allah granted Prophet Musa emancipation from Pharaoh on this day, and so they observed Saum in token of their joy. On this, he (PBUH) said that in this happiness of Prophet Musa, Muslims have a greater right to observe Saum than the Jews. Further destruction of Firaun is celebrated by observing Saum fasting and praise to Allah (SWT).