❖ Question:
If one is unable to fast on the 9th of Muḥarram, should he then fast on the 10th and 11th of Muḥarram instead?
Source: Fatāwā Amunpuri by Shaykh Ghulam Mustafa Zaheer Amunpuri
✿ Answer:
Regarding the fast of ʿĀshūrā’, the prescribed and recommended practice is to fast on both the 9th and 10th of Muḥarram. However, if someone is unable to fast on the 9th, then it is sufficient to fast only on the 10th of Muḥarram.
As for fasting on the 11th of Muḥarram, all narrations regarding it are weak and not reliable as evidence.
❀ Ḥadīth Evidence:
Sayyidunā ʿAbdullāh bin ʿAbbās رضي الله عنهما reported that the Messenger of Allah ﷺ said:
لَئِنْ بَقِيتُ إِلَىٰ قَابِلٍ لَأَصُومَنَّ التَّاسِعَ
"If I live until the next year, I will surely fast on the 9th (of Muḥarram)."
(Ṣaḥīḥ Muslim: 1133)
❀ Statement of Imām al-Nawawī رحمه الله:
Imām al-Nawawī رحمه الله explained:
This ḥadīth proves that it is better to fast on the 9th along with the 10th of Muḥarram, while there is no authentic evidence for fasting on the 11th.
(Sharḥ al-Nawawī ʿalā Muslim: 7/104)

① The Sunnah is to fast on the 9th and 10th of Muḥarram together.
② If one misses the 9th, then fasting on the 10th alone is sufficient.
③ Fasting on the 11th has no authentic basis in ḥadīth.