Voluntary prayer does not become obligatory if a person habituates himself to it

Q: A questioner asks: If a person habituates himself to voluntary prayer, does it become obligatory?
A: No, it does not become obligatory.
Voluntary acts remain voluntary always; they never become obligatory. [Surat al-Baqarah:196]. This is a specific ruling for Hajj and 'Umrah: if one enters ihram for them, they become obligatory until he completes them. As for everything else—such as prayer, fasting, charity, or the like—these voluntary acts remain in their state of being voluntary.
So if he begins a voluntary prayer, it is permissible for him to break it off. If he begins a voluntary fast, it is permissible for him to break it. However, it is better for him to complete it. If he takes out money to give as charity, it is permissible for him to take it back before he hands it over to the poor person. However, it is better not to take back charity.
The point is that all voluntary acts remain voluntary until they are completed, except for Hajj and 'Umrah, for if one begins them, they become obligatory until he completes them.