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) had two appointed muezzins: Bilal and Ibn Umm Maktum (may Allah be pleased with them both), and the latter was blind. The Messenger of Allah (peace and blessings be upon him) said: "Bilal (may Allah be pleased with him) gives the adhan while it is still night, so eat and drink until Ibn Umm Maktum gives the adhan." And there was only as much time between them as it took for one to come down (from the minaret) and the other to go up.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2014