Some scholars have said that nothing is required of him because the vow never took effect; yet the correct view is that the vow is valid, even though it is impermissible to fulfill. A comparable case would be a woman who says, “I have vowed to Allah that I will fast during my menstrual days.” Such a vow is unlawful: she may not fast while menstruating and must instead pay the expiation for breaking an oath.
The third category is the vow of obedience. This is where a person pledges to perform an act of worship—for example, by saying, “I vow to Allah that I will fast the white days, namely the 13th, 14th, and 15th of the month.” He is then obligated to carry out his vow, for the Prophet (peace and blessings be upon him) said, “Whoever vows to obey Allah must obey Him.” The same ruling applies if he says, “I vow to Allah that I will pray two units of the mid-morning (Duha) prayer.” Because the promise involves obedience, he must fulfill it, as the Prophet (peace and blessings be upon him) reiterated: “Whoever vows to obey Allah must obey Him.”
If a single vow contains both obedience and something impermissible, the obedient part must be carried out, whereas the impermissible part must not, and an expiation for breaking an oath is due for the latter. This is illustrated by the man who vowed to stand in the sun, seek no shade, remain silent, and fast. The Prophet (peace and blessings be upon him) commanded him to fast, because that was obedience, but said regarding the standing, avoidance of shade, and silence: “Tell him to sit, to seek shade, and to speak.”
Many people today, when they find a matter difficult or feel anxious about it, make vows. For instance, if someone becomes ill, a person may say, “I vow to Allah that if He cures my loved one, I will do such-and-such.” This practice is prohibited—at the very least disliked and, according to many scholars, forbidden. Ask Allah to heal your sick without attaching a vow. Still, if a vow has in fact been made—stipulating that one will do a certain deed should Allah grant recovery—and the patient is cured, then the vow must be fulfilled. Allah alone grants success.