It is narrated from Sayyidah Unaysah bint Khubayb (may Allah be pleased with her), who had performed Hajj with the Messenger of Allah (peace and blessings be upon him). She said: The Messenger of Allah (peace and blessings be upon him) said: "Bilal calls the adhan while it is still night, so eat and drink until Bilal or Ibn Umm Maktum calls the adhan." One of them would go up and the other would come down, and we would take hold of him and say: Stay as you are, until we have eaten the pre-dawn meal (suhur).
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20851