Question 627: Is it necessary for a person to pass the niyyat for fasting through his mind or to say that he would be fasting on the following day?
Answer 627: Intention (Niyyat) is a condition in fasting, so that a person intends to perform that ‘Ibãdat (devotional service) prescribed by Shari ah, and determines to keep himself away from all those things which invalidate it with the intention of obtaining closeness (to Allah).
If a person intends to abstain from all those things which vitiate a fast, but does not know that some of things have such effect, as, for example, enema, or considers that they have no such effect, but does not practice them, his fast shall be valid.
Similarly, if he intends to abstain from things which he knows are included among those which invalidate the fast, according to the stronger opinion, his fast shall be valid.
In a fast, after the intention of closeness to Allah and sincerity of purpose, there is no further condition in the intention except specifying the fast which one intends to keep in obedience to the command of Allah.
While keeping fast in the month of Ramadãn, it is sufficient to have the intention of keeping fast the next day without specifying it Rather, even if he has an intention of keeping fast other than of Ramadãn in that month due to negligence or forgetfulness, the fast shall be valid, and shall be counted among the fasts of Ramadan, contrary to the one who knows it, so that if he has such intention with knowledge, his fast shall be counted neither as a fast of Ramadãn nor otherwise.
According to the stronger opinion, it is indispensable in case of a fast for other than the month of Ramadàn to specify the particular category of the fast, such as for expiation, compensation or general vow (Nadhr) or a special vow.
It is sufficient to specify briefly, as when it is obligatory on him to keep a special category of fast, and he has the intention to keep fast which he owes, it would be sufficient.
Apparently there is no condition of any specification in a generally recommended fast, so that if a person has the intention of keeping fast the next day for the sake of Allah, it shall be valid, if it is the proper time for it, and the person be one for whom it is valid to keep fast voluntarily.
The same shall be the case if it were a specially recommended fast too, so that it is specified for a particular time, as the blank days (when there is no moon), Friday or Thursday.
Of course, in order to obtain a special reward, it is a condition that the person should keep fast at that particular day and with that special intention.
Sayyid Sistani (ha) says: It is not necessary for a person to pass the niyyat for fasting through his mind or to say that he would be fasting on the following day. In fact, it is sufficient for him to decide that in obedience to the command of Allah he will not perform from the time of Adhan for Fajr prayers up to Maghrib, any act which may invalidate the fast. And in order to ensure that he has been fasting throughout this time he should begin abstaining earlier than the Adhan for Fajr prayers, and continue to refrain for some time after sunset from acts which invalidate a fast.
A person can make niyyat every night of the holy month of Ramadhan that he would be fasting on the following day, and it is better to make niyyat on the first night of Ramadhan that he would fast throughout that month.
For further information in this regards, please refer to the following answers:
Index: Things from which a person keeping fast must abstain & Kaffarah (Expiation), answer 372.
Index: Studying in the month of Ramadhan is not a valid excuse to escape fasting, answer 572.
Index: Using Asthma Inhaler does not break the Fast, answer 575.
Index: Number of things which are disapproved (Makruh) for a person observing fast, answer 566.
Index: Fasting and prayers of a traveler who travel after Zuhr, answer 384.
Index: Qada Fast for a person reverted to Islam, answer 196.
Index: Rules of prayers and fasting performed without ghusl, answer 034.
Index: Fast in countries with long days, answer 106.
 . Tahrir al-Wasilah of Imam Khomeini (ra), Chapter on Intention (Niyyat); Tawzih al-Masael of Maraja’ (with annotation of Imam Khomeini), Vol. 1, Pgs. 881-905.
 . The Official website of the office of Sayyid Sistani (ha), Fasting, Niyyat for Fasting.