Sahih Muslim Book 6, Hadith Number 2407.

Chapter : The timing of fast begins with the dawn.

Samura b. Jandub reported Muhammad (may peace be upon him) as saying. The call of Bilal may not mislead any one of you (and he may, under the wrong impression gathered from it, refrain) from taking meal before the commencement of the fast (for the streaks) of this whiteness (which are vertical indicate the false dawn and the true dawn with which the fast commences is that when the streaks of light are) spread.