‘Abdullāh ibn ‘Umar (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Bilāl calls the Adhān at night; so, eat and drink until you hear the Adhān called by Ibn Umm Maktūm."
The Prophet (may Allah's peace and blessings be upon him) had two Muezzins: Bilāl ibn Rabāh and ‘Abdullāh ibn Umm Maktūm, who was blind. Bilāl (may Allah be pleased with him) used to make the Adhān for the Fajr prayer before the break of dawn, because this prayer occurs at the time of sleep and people need to get ready for it before its time. Hence, the Prophet (may Allah's peace and blessings be upon him) notified his Companions that Bilāl (may Allah be pleased with him) would call the Adhān during the night, and ordered them to eat and drink until the break of dawn, when the second Muezzin, ‘Abdullāh ibn Umm Maktūm (may Allah be pleased with him), would call the second Adhān. This was addressed to whoever intended to fast. So, at that point a person should stop eating and drinking, since the time for prayer would start. This is specific to the Fajr prayer, as it is not permissible in any other prayer to call the Adhān before its due time starts. There is a difference in scholarly opinion concerning the first Adhān for the Fajr prayer: Is it sufficient, or is the second Adhān, which indicates the beginning of the prayer time, necessary? The majority of scholars are of the opinion that the first Adhān is permissible, but not sufficient.