Sahih Bukhari Volume 9, Book 91, Hadith Number 353.

Narrated By Ibn Mas’ud : Allah’s Apostle said, “The (call for prayer) Adhan of Bilal should not stop anyone of you from taking his Suhur for he pronounces the Adhan in order that whoever among you is praying the night prayer, may return (to eat his Suhur) and whoever among you is sleeping, may get up, for it is not yet dawn (when it is like this).” (Yahya, the sub-narrator stretched his two index fingers sideways).