Sahih Bukhari
Fasting
Volume: 003 / Book Number: 031 / Hadith Number: 142
Narrated 'Aisha:
Bilal used to pronounce the Adhan at night, so Allah's Apostle? 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.