How did the Messenger of Allah, may Allah bless him and grant him peace, fast the day of Ashura after his emigration to Medina, and why did he not command the fasting of the ninth day except in the last part of his life?

Fasting is abstaining from things that break the fast, from dawn until sunset, with an intention. The Prophet, peace and blessings be upon him, fasted the day of Ashura in gratitude to Allah for saving Musa and his people from Pharaoh, just as Musa, peace be upon him, fasted it. As for his desire to fast the ninth day, it was to differ from the People of the Book and distinguish the Muslims. After the conquest of Mecca and the spread of Islam, the Prophet, peace and blessings be upon him, was keen to differ from the People of the Book, so he commanded that a day before Ashura or a day after it be added to its fast.