The Prophet (ﷺ) said, "The call (or the Adhan) of Bilal 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.
Sahih al-Bukhari Book 68, Hadith 47