Question:
If someone is unable to fast on the 9th of Muharram, can he fast only on the 10th, the Day of Ashura?
Answer:
To understand this, we must look at how the practice developed.
In the beginning, the fast that was observed was the 10th of Muharram itself. The Prophetﷺfasted on that day and encouraged the Muslims to fast it.
Later, the Prophetﷺexpressed the intention that if he lived until the following year, he would also fast on the 9th of Muharram. The reason he mentioned was to differ from the Jews, who fasted only on the 10th.
From this we understand that fasting the 10th of Muharram alone is valid and acceptable. The fast of Ashura itself remains the primary fast. However, fasting the 9th along with the 10th adds an additional virtue and follows the intention expressed by the Prophetﷺ ﷺ.
Therefore, if a person is unable to fast on the 9th for some reason, he should not think that the fast is lost. He can still fast on the 10th of Muharram, and that fast is valid.
The Prophetﷺalso mentioned the great reward of this fast. He said that fasting on the Day of Ashura expiates the sins of the previous year.
So, the guidance is simple: if possible, fast the 9th and 10th together. But if the 9th is missed, fasting on the 10th alone is still correct and carries great reward.