Question:
There are two hadith about Ashura. One says the Prophetﷺ said, “If I live until next year, I will fast on the ninth day.” He asks if this means to fast only on the ninth, and not the ninth and tenth together, as commonly practiced.
Answer:
This is a common point of discussion. The hadith in Sahih Muslim states that the Prophetﷺ said: “If I remain alive until the next year, I will definitely fast on the ninth [of Muharram].”
When the Prophetﷺ came to Medina, he saw the Jews fasting on the 10th of Muharram. He asked them why. They said it was the day Allahﷻ saved Prophetﷺ Musa (Moses) and the Children of Israel from Pharaoh, and Musa (AS) fasted on that day in gratitude. The Prophetﷺ said, “We have more right to Musa than you.” So he fasted on that day and commanded others to fast.
Later, he expressed a desire to be different from the Jews. He said he would fast on the 9th of Muharram as well. The hadith only mentions fasting on the 9th. However, the context is crucial. The reason for the fast is to commemorate the salvation of Musa (AS) and to show gratitude to Allahﷻ. That event happened on the 10th of Muharram.
If the Prophetﷺ had intended to abandon fasting on the 10th entirely, he would have been abandoning the commemoration of Musa (AS), to whom he said Muslims are more entitled. Therefore, the correct understanding is that he intended to add the 9th to the 10th, to be different from the Jews who only fasted on the 10th. He wanted to fast on both the 9th and the 10th, or to at least include the 9th. This is the interpretation of the majority of scholars.
The hadith that clearly states the Prophetﷺ fasted on the 10th and commanded it and then expressed the intention to fast on the 9th, shows that the 10th remains a significant day. The change was to add the 9th, not to replace the 10th. Therefore, the Sunnah is to fast on both the 9th and 10th of Muharram, or at least to include the 9th along with the 10th. Fasting only on the 9th would mean missing the primary day of the event.