Chapter on the Rulings of Vows
Linguistically, a vow (nadhr) means to impose an obligation upon oneself. You say, “I vowed such-and-such” when you have made it binding upon yourself.
In the terminology of the Sacred Law, it is: for a legally responsible person, acting by choice, to obligate himself to perform something for the sake of Allah Most High.
A vow is a form of worship, and it is impermissible to direct any act of worship to other than Allah. Thus, whoever makes a vow to a grave, an angel, a prophet, or a saint has committed major shirk that expels one from the religion, because in so doing he has worshipped someone besides Allah. Those who today place vows at the tombs of the saints and the righteous have therefore fallen into the gravest form of association with Allah—may Allah protect us. It is incumbent upon them to repent to Allah, to beware of this practice, and to warn their people so that they may take heed.
The basic ruling of initiating a vow is that it is disliked, and a number of scholars have even declared it forbidden. This is on account of the hadith narrated by Ibn ʿUmar (may Allah be pleased with them both) in which the Prophet (peace and blessings be upon him) forbade vows, saying, “They do not avert anything; they are only a means by which something is extracted from a miser.” Al-Muntaqā comments: “It was related by all the compilers except al-Tirmidhi.”
Moreover, the one who makes a vow is binding himself to something that was not originally obligatory for him.