Combining Intention of Fasting Ramadan With A Vow or A Voluntary Fast
5-8-2015 | IslamWeb
Question:
I promised Allah Almighty not to return to an act of disobedience that I used to commit. I promised Him that if I returned, I would fast for a hundred days. I asked a Mufti about whether I could feed 60 needy people instead of fasting a hundred days, and he told me that I should fast if I am able. I am a healthy young man, and I have the ability to fast, praise be to Allah Almighty. My question is: may I, when fasting in Ramadan, make a double intention (observing the obligatory fast of Ramadan and the fast of the vow)? Concerning fasting the six days of Shawwal, the white days, the days of ‘Arafah and ‘Ashoora’ and the other days whose fasting is a Sunnah, may I combine the intention of fasting them with the intention of fasting for the vow?
Answer:
All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad 
is His slave and messenger.
The Muslim should avoid acts of disobedience in all cases out of fear of Allaah Almighty and His punishment. If the promise you gave to Allaah Almighty to abandon this act of disobedience was only an intention in your heart and you did not explicitly pronounce anything that indicates commitment to fasting in case you returned to this act of disobedience, then you are not committed to anything according to the opinion of the majority of the scholars. This is based on the fact that mere intention does not render a vow binding. Also, a vow should be explicitly pronounced in a statement that indicates commitment, like by saying, for example, 'it is due to Allaah Almighty upon me to do such-and-such'. It was authentically narrated that the Prophet said: “Allaah Almighty overlooks the thoughts of the members of my community unless they bring them to action or talk about them.”
However, if you explicitly stated a commitment to fast for a hundred days if you returned to this act of disobedience, then that is a vow, and it is called a vow of lajaj (i.e. anger). What you should do concerning it is one of two matters: either fulfill your vow by fasting a hundred days or offer expiation for breaking an oath; feeding ten needy people, clothing them, or freeing a slave. If you cannot do any of the three, then you should fast for three days. This is the preponderant opinion among the scholars. Shaykh Al-Islam Ibn Taymiyyah 
said: "According to the prominent opinion, a vow of lajaj entails one of two matters: either expiation or fulfilling the vow. There is no doubt that statements like, ‘if I do such-and-such thing, then I have to pray two Rak‘ahs (units of prayer), pay a thousand in charity, perform Hajj, or fast for a month’ entail obligation. In that case, the person is given the choice between fulfilling this obligation or offering the obligatory expiation. If he does not adhere to the conditional obligation, then he is committed to the obligation of expiation. He is bound to one of the two obligations. Each of them is obligatory if the other is not realized. This resembles the obligation in which one is given the choice between two options."
If you wish to fulfill your vow on the basis that it is obligatory, then you should fast a hundred days with the intention of fulfilling it. You are not allowed to combine the intention of fasting those days with the intention of fasting the month of Ramadan. Moreover, you should not combine the intention of fasting for the vow with the intention of fasting the six days of Shawwal, the white days, and the like.
Allaah Knows best.