It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Bilal calls the adhan while it is still night, so keep eating and drinking until Ibn Umm Maktum calls the adhan."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13050