Fasting is a shield for a person which protects them from Shaytan, Allah’s punishment and Jahannam. However, one needs to make sure the shield is not damaged in any way. Otherwise it will not be effective in doing its job. The actions that damage this shield and render it useless are sins like Backbiting, Lying, Evil Glance, Swearing, Nonsensical Conversation, Arguments, Slander, Haram Sustenance, and every other evil.
Besides the compulsory fasting in the month of Ramadan, one should try to fast during those days for which Rasoolullah (صلي الله عليه و سلم) has mentioned many rewards, for example:
- 6 days of Shawwal
- Day of Aarafah
- Ashoora (9th & 10th or 10th & 11th of Muharram)
Source: Riyadul Jannah Issue 2 Vol 13