Question 17: What is the religious ruling on observing Sawm (Fasting) on the last Ten Days of Thul-Hijjah and the whole of the months of Muharram and Sha‘baan? Answer us, may Allaah confer His Blessings upon you!
Answer: Bismillaah [In the Name of Allaah] and all praise is due to Him. It is Mashroo‘ (Islaamically permissible) to fast in the month of Muharram and Sha‘baan. As for the last Ten Days of Thul-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.The Prophet (peace be upon him) said concerning the month of Muharram: The best Sawm after Ramadaan, is the Month of Allaah: 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... read more here.