Sahih Bukhari Volume 7, Book 63, Hadith Number 219A.

Narrated By ‘Abdullah bin Mas’ud : The Prophet said, “The call (or the Adhan) of Bila should not stop you from taking the Suhur-meals for Bilal calls (or pronounces the Adhan) so that the one who is offering the night prayer should take a rest, and he does not indicate the daybreak or dawn.” The narrator, Yazid, described (how dawn breaks) by stretching out his hands and then separating them wide apart.