Question: When does the one who (intends) to fast stop eating and drinking; it is whilst the Adhaan (of Fajr) is being called or after the Adhaan has been called?
Answer: The Islamic legislation commands cessation of (eating and drinking) when fajr arrives (i.e. when a person is certain that Fajr has entered), whether before the Adhaan or after the Adhaan (i.e. whether he hears the Adhaan being called after he is already certain that fajr has entered or whether he heard it thereafter after already being certain that it has entered). That is because Allaah said:
“And eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night), then complete your Saum (fast) till the nightfall.”
[Surah Al-Baqara: Ayah: 187]
However, the common practice is that the Mu-adh-dhin does not call the Adhaan except when fajr arrives. So when it is the case that it has being legislated to (cease eating and drinking) when the Adhaan is called, then it is incumbent to stop and not eat. Therefore, resolve to stop eating when the Adhaan begins, because in the majority of cases the Mu-adh-dhin does not call the Adhaan except when fajr arrives, especially the Mu-adh-dhin of Masjid Al-haraam (i.e. the Kabah).
[Source: Al-Fataawaa Wad-Duroos Fil-Masjid Al-Haraam of Shaikh Abdullaah Bin Muhammad Bin Humaid (rahimahullaah) page:473-474]