Call to Prayers (Adhaan) 
Bukhari :: Book 1 :: Volume 11 :: Hadith 594
Narrated 'Abdullah bin 'Umar:
Allah's Apostle said, "Bilal pronounces the Adhan at night, so keep on eating and drinking (Suhur) till Ibn Um Maktum pronounces the Adhan."
Call to Prayers (Adhaan)
Bukhari :: Book 1 :: Volume 11 :: Hadith 594
Narrated 'Abdullah bin 'Umar:
Allah's Apostle said, "Bilal pronounces the Adhan at night, so keep on eating and drinking (Suhur) till Ibn Um Maktum pronounces the Adhan."