Sayyiduna Ibn Umar and Sayyidah Aishah (may Allah be pleased with them) say that the Messenger of Allah (peace and blessings be upon him) said: "Bilal calls the adhan while it is still night, so eat and drink until you hear the adhan of Ibn Umm Maktum."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8022