Once the entire disk of the sun has disappeared, the fasting person should break his fast, and not pay any attention to the red glow that remains on the horizon, because the Prophet (peace and blessings of Allaah be upon him) said: "Once night comes from there and the day disappears from there, and the sun has set, the fasting person should break his fast." (Reported by al-Bukhaari, al-Fath, no. 1954; the issue is also mentioned in Majmoo’ al-Fataawa, 25/216).
The Sunnah is to hasten in breaking the fast. The Prophet (peace and blessings of Allaah be upon him) would not pray Maghrib until he had broken his fast, if only with a sip of water. (Reported by al-Haakim, 1/432; al-Silsilat al-Saheehah, 2110). If a fasting person cannot find anything with which to break his fast, he should have the intention in his heart to break his fast, and he should not suck his finger, as some of the common people do. He should beware of breaking the fast before the correct time, because the Prophet (peace and blessings of Allaah be upon him) saw some people hanging from their hamstrings with blood pouring from the corners of their mouths, and when he asked about them, he was told that they were people who broke their fast before it was time to do so." (The hadeeth is in Saheeh Ibn Khuzaymah, no. 1986, and in Saheeh al-Targheeb, 1/420). If a person is certain, or thinks it most likely, or is not sure whether he broke the fast before the proper time, he should make up the fast later on, because the basic principle is that the day is still there and has not ended. (Fataawa al-Lajnah al-Daa’imah, 10/287). He should beware of relying on the word of small children or untrustworthy sources, and he should also beware of the time differences between different cities and villages when he hears the adhaan on the radio and so on.
When the dawn comes – which is the white light coming across the horizon in the East – the fasting person must stop eating and drinking straightaway, whether he hears the adhaan or not. If he knows that the muezzin calls the adhaan at dawn, he has to stop eating and drinking as soon as he hears his adhaan, but if the muezzin calls the adhaan before Fajr, he does not have to stop eating and drinking when he hears it. If he does not know the muezzin’s usual practice, or there are differences among the muezzins, and he cannot determine the time of dawn for himself – as is usually the case in cities because of lighting and buildings – he should take the precaution of referring to a printed timetable, so long as he is sure that the calculations on which it is based are not incorrect.
The idea of being on the safe side by stopping eating and drinking a certain time before Fajr, such as ten minutes before, is bid’ah. On some timetables you can see one heading for "imsaak" (stopping eating and drinking) and another for Fajr; this is something that is contrary to Islam.
The Muslims living in cities where there is a distinct alternation of night and day in every twenty-four hour period are obliged to fast, no matter how long the day is, so long as that distinction between night and day is there. In some places there is no such distinction between night and day; Muslims in these places should fast according to the times in the nearest city in which there is a distinct alternation of night and day.