Is it permissible to break the fast before hearing the adhaan?

When the sun
has set, it becomes permissible for the fasting person to break the fast,
whether the muezzin has given the adhaan or not. What counts is the setting
of the sun, because the Prophet (peace and blessings of Allaah be upon
him) said: “When the night comes from here and the day departs from here,
and the sun has set, then it is time for the fasting person to break his
fast.” Narrated by al-Bukhaari, 1954; Muslim, 1100. 

Ibn Daqeeq
al-‘Eid said: This hadeeth is a refutation of the Shi’a who delay breaking
the fast until the stars appear. End quote from Fath al-Baari.

 Some
muezzins may delay the adhaan until a while after the sun has set, so the
adhaan is not what counts. This action of theirs is contrary to the guidance
of the Prophet (peace and blessings of Allaah be upon him) who urged us
to hasten to break the fast after the sun has set. He said: “The people will
still be fine so long as they hasten to break the fast.” Narrated by
al-Bukhaari, 1957; Muslim, 1098. It is permissible for the fasting person to
break the fast if he thinks it most likely that the sun has set; it is not
essential that he be certain, rather thinking it most likely is sufficient. 

If the
fasting person thinks it most likely that the sun has set, and he breaks his
fast, there is no sin on him. But it is not permissible for him to break his
fast if he is not sure whether the sun has set. 

Shaykh Ibn
‘Uthaymeen (may Allaah have mercy on him) said: 

It is Sunnah
to hasten to break the fast when the sun has set. What counts is the setting
of the sun, not the adhaan, especially in present times when the people rely
on timetables and they follow the timetables based on the clocks, and their
clocks may be fast or slow. If the sun sets and you see it, and the people
have not yet given the adhaan, then you may break the fast. If they give the
adhaan and you see that the sun has not set, then you should not break the
fast, because the Messenger (peace and blessings of Allaah be upon him)
said: “When the night comes from here and the day departs from here, and the
sun has set, then it is time for the fasting person to break his fast.” 

It does not
matter if there is still strong light. Some people say: We will carry on
fasting until the disk of the sun has disappeared and it has gotten somewhat
dark. But this does not matter; rather you should look at the disk of the
sin and when the top of it disappears, then the sun has set, and it is
Sunnah to break the fast. 

The evidence
that it is Sunnah to break the fast at this time is the words of the Prophet
(peace and blessings of Allaah be upon him): “The people will still be
fine so long as they hasten to break the fast.” Hence we know that those who
delay breaking the fast until the stars appear, like the Raafidis, are not
fine. 

If someone
were to say: Can I break the fast when I think it most likely that the sun
has set?  

The answer
is yes, and the evidence for that is the report that is narrated in
Saheeh al-Bukhaari from Asma’ bint Abi Bakr (may Allaah be pleased with
her) who said: We broke our fast on a cloudy day at the time of the Prophet
(peace and blessings of Allaah be upon him), then the sun appeared. It
is known that they did not break their fast on the basis of certain
knowledge, because if they had broken their fast on the basis of certain
knowledge the sun would not have appeared after that; rather they broke
their fast on the basis of their thinking it most likely that it had set,
then the clouds dispersed and the sun appeared. End quote. 

Al-Sharh
al-Mumti’, 6/267.