Ruling on one who breaks the fast by mistake

Al-Bukhaari (6669) and Muslim (1155) narrated that Abu
Hurayrah (may Allaah be pleased with him) said: The Prophet (peace and
blessings of Allaah be upon him) said: “Whoever forgets he is fasting and
eats or drinks, let him complete his fast for it is Allaah Who has fed him
and given him to drink.” 

It was also narrated that he does not have to offer expiation
or make up that fast. 

Ibn Khuzaymah (1999) narrated from Abu Hurayrah that the
Prophet (peace and blessings of Allaah be upon him) said: “Whoever
breaks his fast in Ramadaan by mistake does not have to make up that day or
offer expiation.” Classed as hasan by al-Albaani in Saheeh Ibn Khuzaymah. 

Al-Daaraqutni narrated
from Abu Sa’eed al-Khudri that the Prophet (peace and blessings of
Allaah be upon him) said: “Whoever eats in the month of Ramadaan by mistake
does not have to make up that day.” 

Al-Haafiz said: 

Although its isnaad is weak, it may still be considered sound
because there are corroborating reports. The least that could be said about
it is that it is hasan, so it may be quoted as evidence. Reports that are
less strong than this have been quoted as evidence with regard to many
issues. It may also be supported by the fact that a number of the Sahaabah
issued fatwas that are in agreement with this hadeeth without any one of the
Sahaabah having a different view, as was stated by Ibn al-Mundhir, Ibn Hazm
and others, and ‘Ali ibn Abi Taalib, Zayd ibn Thaabit, Abu Hurayrah, and Ibn
‘Umar. And it is in accordance with the words of Allaah (interpretation of
the meaning): 

“but He will call you to account for that which your
hearts have earned”

[al-Baqarah 2:225] 

Forgetfulness is not something which is earned by the heart. 

This hadeeth tells of the kindness of Allaah to His slaves
and how He makes things easier for them and alleviates hardship. 

End quote. 

The majority of scholars quote these ahaadeeth as evidence
that whoever forgets that he is fasting and breaks the fast, his fast is
still valid, and he should complete his fast, and he does not have to make
it up or offer any expiation. The general meaning of the hadeeth covers both
obligatory and naafil fasts; there is no difference between the two. 

Al-Shaafa’i said in al-Umm (2/284): 

If a fasting person eats or drinks in Ramadaan or in a fast
observed in fulfillment of a vow or as an expiation, or a fast that is
obligatory for some reason, or a voluntary fast, out of forgetfulness, then
his fast is complete and he does not have to make it up.  

Al-Nawawi said: 

This is evidence to support the view of the majority: if the
fasting person eats or drinks or has intercourse because of forgetfulness,
then he does not break his fast. Among those who were of this view are
al-Shaafa’i, Abu Haneefah, Dawood and others. 

Al-Haafiz said: 

An interesting story was narrated by ‘Abd al-Razzaaq from
‘Amr ibn Dinar: that a person came to Abu Hurayrah and said: “I started
fasting in the morning then I forgot and ate.” He said, “It does not
matter.” He said: “Then I entered upon someone and by mistake I ate and
drank.” He said, “It does not matter, Allaah has fed you and given you to
drink.” Then he said:  “I entered upon another person and forgot, and ate.”
Abu Hurayrah said: “You are a person who is not used to fasting.”.