Sayyidah Unaysah (may Allah be pleased with her), who prayed a great deal, narrated from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "Ibn Umm Maktum or Bilal calls the adhan while it is still night, so eat and drink until Bilal or Ibn Umm Maktum calls the adhan." As soon as one of them had called the adhan, the other would climb up, and we would take him by the hand and say: Stay as you are, until we have eaten the pre-dawn meal (suhur).
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20852