What is the ruling on vowing to fast five days of every month for the healing of one's father, without specifying a duration, and what are the implications of being unable to fulfill it?

Whoever makes a vow of obedience must fulfill it, for Allah has praised those who fulfill their vows. Allah says: (They fulfill [their] vows and fear a Day whose evil will be widespread) and Allah says: (And let them fulfill their vows). The Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him; and whoever vows to disobey Allah, let him not disobey Him."

However, if an excuse arises, such as illness or continuous inability, the vow is waived for the one who made it, and he must offer an expiation for breaking an oath. If the excuse is removed, he must return to fulfilling his vow.