Making A Promise to Allah

Making a promise to Allah is a kind of vow (nadhr). Allah has commanded us to fulfil our promises and covenants, whether the promise is made to Allah or to people. Allah says (interpretation of the meaning):

“And fulfil the Covenant of Allaah (Bay‘ah: pledge for Islam) when you have covenanted”
[al-Nahl 16:91]

Allah says :

“And of them are some who made a covenant with Allah (saying): ‘If He bestowed on us of His Bounty, we will verily, give Sadaqah (Zakaah and voluntary charity in Allah’s Cause) and will be certainly among those who are righteous’”
[al-Tawbah 9:75]

Abu Bakr al-Jassaas said in Ahkaam al-Qur’aan (3/208) concerning this verse:

"This indicates that whoever makes a vow to observe an act of worship has to fulfil it, because a promise or covenant is a vow and an obligation."

This applies if what the person wants to do is an act of worship, such as when a person makes a promise to his Lord that he will speak the truth or give some of his wealth in charity, etc. This comes under the heading of vows (nadhr) even if the person does not actually use the word nadhr or vow. The one who makes a promise to Allah or says, “I promise Allah that I will do such and such” is obliged to fulfill it, because the Prophet (peace and blessings of Allah be upon him) said: “Whoever vows to do an act of worship or obedience to Allah, let him fulfill it, and whoever vows to do an act of disobedience towards Allah, let him not fulfill it.”
Narrated by al-Bukhaari, 6318

It is not permissible to fail to fulfill this obligation, unless a person is unable to fulfil it at all, in which case he should offer kafaarat yameen (expiation for breaking a vow), because of the hadeeth:

“Whoever makes a vow and is unable to fulfil it, his expiation is kafaarat yameen.”

Narrated by Abu Dawood, 3322, from the hadeeth of Ibn ‘Abbaas. Al-Haafiz said in al-Fath: Its narrators are thiqaat, but it was narrated by Ibn Abi Shaybah in a mawqoof report, which is likely to be the case.

Ibn Qudaamah said in al-Mughni (10/72): In conclusion: the one who vows to do an act of worship but is unable to do it, or was able to do it then became unable to do it, has to offer kafaarat yameen.

The expiation for breaking an oath is to set free a slave, or to feed or clothe ten poor persons. Whoever cannot do any of those things should fast for three days, because Allah says:

“Allah will not punish you for what is unintentional in your oaths, but He will punish you for your deliberate oaths; for its expiation (a deliberate oath) feed ten Masaakeen (poor persons), on a scale of the average of that with which you feed your own families, or clothe them or manumit a slave. But whosoever cannot afford (that), then he should fast for three days. That is the expiation for the oaths when you have sworn. And protect your oaths (i.e. do not swear much). Thus Allah makes clear to you His Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.) that you may be grateful”
[al-Maa'idah 5:89]


SOURCE:
Islamqa Fatwa no. 38934