When observing a specific supererogatory fast, it is stipulated that the intention should be formed from the night before

Question

Praise be to Allah.

In the case of a supererogatory fast
that is not done for a specific reason, it is not stipulated that the
intention should be formed from the night before; rather if a person
decides to fast during the day, then he fasts until sunset, that is
acceptable, so long as he has not done anything that breaks the fast since
dawn broke.

But in the case of a supererogatory fast
that is done for a specific reason, it is essential to form the intention
from the night before (i.e., from before dawn).

Shaykh Ibn ‘Uthaymeen (may Allah have
mercy on him) was asked:

Does fasting six days of Shawwaal and the
day of ‘Arafaah come under the same ruling as obligatory fasts, in that it
is stipulated that the intention should be formed from the night before? Or
does it come under the same ruling as general supererogatory fasts; it is
permissible for the individual to intend to fast that day, even if that is
halfway through the day? Will the reward of one who decides to fast halfway
through the day be like the reward of the one who eats sahoor and fasts from
the beginning to the end of the day?

He replied:

Yes, it is permissible to intend to
observe a supererogatory fast partway through the day, on condition that one
has not done any action that breaks the fast before that. For example, if a
person eats after dawn has broken, then during the day he decides to fast,
we would say to him in that case: Your fast is not valid, because you ate
something. But if he had not eaten anything from the time dawn broke, and he
did not do anything that breaks the fast, then he decided during the day to
observe a supererogatory fast, then we say: this is acceptable, because it
is narrated that the Prophet (blessings and peace of Allah be upon him) did
that.

The reward is only from the time the
intention was formed, because the Prophet (blessings and peace of Allah be
upon him) said: “Actions are but by intentions.” So whatever came before the
intention was formed will have no reward recorded for it, but reward will be
recorded for what comes after it.

If the reward is connected to the day’s
fast, then he has not fasted the entire day; rather he has fasted part of
the day with the intention. Based on that, if someone were to get up after
dawn breaks and not eat anything, then halfway through the day he decided to
fast on the basis that it was one of the six days, then he fasted five more
days after that, then he will have fasted five and a half days. If he formed
the intention to fast after one quarter of the day had passed, then he will
have fasted five and three quarters days, because actions are judged by
intentions, and the hadith says: “Whoever fasts Ramadan then follows it with
six days of Shawwaal.”

In that case we say to this brother: you
have not attained the reward for fasting the six days, because you did not
fast six full days. The same may be said with regard to the day of ‘Arafah.
But if the fast is a general supererogatory fast (not connected to a
specific reason), then it is valid and he will be rewarded from the time he
formed the intention only.

End quote from Liqa’ al-Baab al-Maftooh
(55/21)

He also said:

If the virtue of the fast is connected to
a particular day, such as fasting on Mondays and Thursdays, fasting the
ayyaam al-beed (the 13th, 14th and 15th of
the Hijri month), fasting three days of every month, and he forms the
intention (to fast) during the day, then he does not attain the reward of
that day.

So for example, if he fasts on a Monday
but forms the intention to fast during the day, he will not earn the same
reward as one who fasted from the beginning of the day, because it cannot
truly be said of him that he fasted (all of) Monday.

Similarly, if a person wakes up not
fasting, then he is told that today is the thirteenth of the (Hijri) month,
and it is the first of al-ayyaam al-beed, so he said: Then I am fasting, he
will not earn the reward of al-ayyaam al-beed, because he has not fasted an
entire day.

End quote from ash-Sharh al-Mumti‘
(6/360)

For more information, please see the
answer to question no. 21819

And Allah knows best.