Short answer
Yes, the fast is valid and rewarded, but the Hanafi school says fasting the 10th alone is makruh tanzihi. The sunnah is to add the 9th, or the 11th if you missed the 9th, as the Prophet ﷺ intended to do. The Shafi'i and Hanbali schools see no dislike in fasting the 10th alone, though they too prefer adding the 9th.
Fasting Ashura (10 Muharram) is mustahabb. Fasting it alone is makruh tanzihi, because the Prophet ﷺ intended to fast the 9th as well; adding the 9th or the 11th removes the dislike. The best is to fast the 9th and the 10th.
Other madhhabs
- Shafi'i
- Fasting the 10th alone is not disliked; adding the 9th, and also the 11th, is recommended.
- Hanbali
- Fasting the 10th alone is not disliked in the main view of the school, though fasting the 9th with it is better.
Explanation
The Prophet ﷺ said that fasting the day of Ashura, he hoped from Allah, would expiate the year before it. Near the end of his life he said that if he lived to the next year he would also fast the 9th, but he passed away before then. The scholars understand that the 9th was added to differ from the People of the Book, who fasted the 10th alone.
Your options, from best:
- The 9th and 10th of Muharram.
- The 10th and 11th, if you missed the 9th.
- The 9th, 10th and 11th, which some scholars mention as the fullest.
- The 10th alone: valid and rewarded, but mildly disliked in the Hanafi school.
The dislike is mild. If you can only manage one day, fasting the 10th alone is far better than not fasting at all, and you still hope for the expiation in the hadith. Some later Hanafi scholars also held that the dislike is weaker today, since the reason for it is less present, but adding a day remains the sunnah.
For the date of Ashura this year and its history, see Ashura. For other voluntary fasts, see Mondays, Thursdays and the white days.
The evidence
Sahih Muslim 1162
Sahih... صيام يوم عاشوراء أحتسب على الله أن يكفر السنة التي قبله ...
"Fasting the day of Ashura, I hope from Allah, expiates the year before it."
The virtue of fasting Ashura.
Sahih Muslim 1134
Sahih... لئن بقيت إلى قابل لأصومن التاسع ...
"If I live until next year, I will certainly fast the ninth."
The basis for adding the 9th to the 10th.
Where the ruling is recorded
- al-Durr al-Mukhtar, Kitab al-Sawm, section on the kinds of fasts. Fasting Ashura alone is among the makruh tanzihi fasts.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Sawm, section on the kinds of fasts. The dislike is lifted by joining the 9th or the 11th to it.
- al-Mughni (Ibn Qudama), Kitab al-Siyam, section on Ashura. The Hanbali view that fasting the 9th and 10th is recommended.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.