The days permissible for voluntary (fast) Sawm

Question: I am a 27 year-old Saudi man. I went to prison and I began to worship Allaah. I observe Sawm (Fast) on Monday and Thursday of every week; on three days of every month; the whole month of Rajab; nine days of Thul-Hijjah; the Day of `Ashoorah’ (10th of Muharram) and a day before and after it; six days of Shawwaal; and mid-Sha‘baan. It is claimed that Sawm should be observed in Ramadaan only, and the rest is a Bid‘ah (innovation in religion), and there is no Saheeh (authentic) Hadeeth that proves them. But I found a Saheeh Hadeeth which states this in the book entitled Tanbeeh Al-Ghaafileen by Shaykh Aboo Al-Layth Al-Samarqandee. Please advise whether Sawm on these days is really a Bid‘ah, given that my colleagues in prison say so.

Answer: Sawm on Mondays and Thursdays, three days of every month (13th, 14th, and 15th of every Hijree month), the ninth of Thul-Hijjah, the tenth of Muharram, in addition to a day before or after it, and six days of Shawwaal, every year, all this is Sunnah (supererogatory acts of worship following the example of the Prophet) established by authority of Saheeh Hadeeths... read more here.