Musnad Ahmad 1153:264

Yahya narrated to us, from Ubaydullah, who said: I heard al-Qasim, from Aisha, from the Prophet, peace and blessings of Allah be upon him: "Indeed, Bilal gives the adhan at night, so eat and drink until Ibn Umm Maktum gives the adhan." He said: "And I do not know it to be anything other than the amount of time it takes for this one to descend and this one to ascend."