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." And he stated that Bilal would call the adhan, then when Bilal came down (after calling the adhan), Ibn Umm Maktum would call the adhan.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13322