Sahih Al Bukhari Hadith Number 1919

Narrated `Aisha: Bilal used to pronounce the Adhan at night, so Allah’s Messenger (peace be upon him)? said, “Carry on taking your meals (eat and drink) till Ibn Um Maktum pronounces the Adhan, for he does not pronounce it till it is dawn.

30  Fasting
Hadith number 1919
Volume no 3
Chapter: 17 (The Adhan of Bilal should not stop you from taking Sahur”)