It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: Bilal does not know how much of the night remains. Therefore, keep eating and drinking until Ibn Umm Maktum gives the call to prayer.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 6050
Hadith Gradingحکم دارالسلام:إسناده صحيح ، وقوله "إن بلالا لايدري ما الليل" لم أجده في غير هذا الموضع.