Reward for the one who misses an optional act worship due to sickness, travel or a legislated shariah excuse
Does a person become sinful due to failing to fulfill acts of obedience (to Allaah)? And what is obligated on a person if he (or she) misses an act of worship which he (or she) is determined to perform, even though he (or she) was prevented from performing it due to illness or a legislated shariah excuse?
If a Muslim always performs an act of obedience to Allaah when he (or her) is at home, but then he is prevented from performing that act (of obedience) due to a legislated excuse, he (or she) will be rewarded even if he (or her) did not perform it. And this is due to the hadeeth reported by Abu Moosaa (radiyallaahu-anhu) that Messenger said: If a slave of (Allah) becomes sick or travels, he (or she) is rewarded for the good deeds he (or she) used to perform when he (or she) was healthy or at home.
As for the acts of obedience he (or she) abandons whilst having the ability to perform them, he (or she) will be deprived of their reward. And this may lead to sin and acts of disobedience (i.e. if obligations are abandoned). Therefore, it is incumbent upon a person that he (or she) repents to Allaah, seeks Allaah’s forgiveness often and be eager to perform acts of obedience to Allaah, so that Allaah may aid him (or her).
[Source: Fadaa’il Al’A’maal page 8’ by Shaikh Muhammad Bin Abdillaah As-Subayyil (rahimahullaah)]