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.
تيسير العلام شرح عمدة الأحكام للبسام، حققه وعلق عليه وخرج أحاديثه وصنع فهارسه: محمد صبحي بن حسن حلاق، ط10، مكتبة الصحابة، الإمارات - مكتبة التابعين، القاهرة، 1426هـ. تنبيه الأفهام شرح عمدة لأحكام لابن عثيمين، ط1، مكتبة الصحابة، الإمارات، 1426هـ. الإلمام بشرح عمدة الأحكام لإسماعيل الأنصاري، ط1، دار الفكر، دمشق، 1381هـ. صحيح البخاري، تحقيق: محمد زهير بن ناصر الناصر، ط1، دار طوق النجاة (مصورة عن السلطانية بإضافة ترقيم: محمد فؤاد عبد الباقي)، 1422هـ. صحيح مسلم، تحقيق: محمد فؤاد عبد الباقي، دار إحياء التراث العربي، بيروت.