Musnad Ahmad 1280:3

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Khubayb ibn ‘Abd al-Rahman, from his paternal aunt, who said: The Prophet, peace and blessings of Allah be upon him, said: "Indeed, Ibn Umm Maktum—or Bilal—calls the adhan at night, so eat and drink until Bilal—or Ibn Umm Maktum—calls the adhan." She said: It would be nothing but that one of them would call the adhan, and the other would ascend, so we would take him by the hand and say, "Stay as you are until we finish our suhur."