At-Tabaraani (R.A) reported that Ibn ‘Abbaas (R.A) narrated that the Prophet (S.A.W) said: “Whoever fasts one day during the month of Muharram will get a reward of fasting 30 days.” This is a fabricated narration as Shaykh Al-Albaani (R.A) classified it.
However, what is confirmed regarding the excellence of fasting during the month of Muharram is what is reported by Imaam Muslim (R.A) that the Prophet (S.A.W) said: “The best fasting after the month of Ramadhaan is during the month of Muharram, and the best prayer after the obligatory prayer is the optional night prayer.”
There are other narrations that encourage fasting during all the sacred months (i.e. Rajab, Thul-Qi’dah, Thul-Hijjah, Muharram).
As regards how many days we fast in Muharram, it is recommendable to fast the whole of Allaah’s month Muharram for the aforementioned Hadeeth narrated by Muslim (R.A).
However, if it is difficult to fast the full month, the person can fast the tenth day of Muharram (the day of ‘Aashooraa).
There are three ways of fasting it:
1) This one is more perfect: fasting the tenth of Muharram, the day before it and the day after it.
2) Fasting the tenth of Muharram and the day before it (or the day after it).
3) Fasting the tenth of Muharram only.
Allah knows every thing well.