Fasting is ibadah
As fasting is an act of ‘ibādah in itself, just keeping the fast means you are in a state of ‘ibādah. If you can do any other ‘ibādah at the same time, that is very good. If you cannot do any proactive ‘ibādah you should not feel guilty about it as you are already in a state of ‘ibādah. However, it is important that the fast we keep adheres to the spirit of fasting.
(Excerpt from a Spiritual Gathering held by Shaykh Riyadh ul Haq)