The Voluntary Fast is not made up for
Question :
I fast three days of every month and during one of the months I got sick, so I did not fast them. Am I obligated to make up for the days or make expiation?
Answer:
The voluntary fast is not made up for, even if it was left off by choice. However, it is better for the Muslim to be consistent in what he does of righteous deeds. This is due to the Hadith:
"The most beloved of deeds to Allah are the most consistent of them, even if they are few."
Thus, you do not have to make up for the fast or make expiation, while knowing that whatever the person left off of righteous deeds that he used to do because of illness or inability or travel and similar things, the reward of it is still written for him. This is due to the Hadith:
"If the son of Adam becomes ill or travels, whatever he used to do while he was healthy and at his home is written for him (i.e., as if he did it)."
Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 3 Pages 339-340