Q: What is the religious ruling on observing Sawm (Fasting) on the last Ten Days of Dhul-Hijjah and the whole of the months of Muharram and Sha‘ban? Answer us, may Allah confer His Blessings upon you!  |
A: Bismillah [In the Name of Allah] and all praise is due to Him. It is Mashru‘ (Islamically permissible) to fast in the month of Muharram and Sha‘ban. As for the last Ten Days of Dhul-Hijjah, there is no evidence concerning fasting on them; but there is nothing wrong with fasting on them without having the belief that they have certain specialty or fasting is recommended on them in particular.
| (Part No. 15; Page No. 416) |
The Prophet (peace be upon him) said concerning the month of Muharram: 
The best Sawm after Ramadan, is the Month of Allah: Muharram.
Therefore, it is recommendable to fast the whole of Muharram and it is Sunnah (supererogatory act of worship following the example of the Prophet) to fast on the ninth, the tenth and the eleventh of it.
Similarly, it is reported that the Prophet (peace be upon him) used to fast the entire of Sha‘ban or most of it as authentically reported in the Hadith of `Aishah and Um Salamah (may Allah be pleased with both of them).
With regard to fasting the Ten Days of Dhul-Hijjah, first of all what is meant is the first nine days, as the Day of ‘Eid is not allowable to fast on, but it is recommendable to fast on these nine days for it brings a great reward according to the general meaning of the Hadith of the Prophet (peace be upon him),
There are Hadiths that indicate that the Prophet (peace be upon him) used to fast on these days and others that show that he (peace be upon him) did not fast on them, but nothing has been authentically reported that determines his fasting or not fasting them.
Source: Fatwas of Ibn Baz