Sahih Muslim Book 6, Hadith Number 2410.
Chapter : The timing of fast begins with the dawn.
Samura b. Jundub addressed and narrated from the Apostle of Allah (may peace be upon him) having said (these words): Neither the call of Bilal should mislead you nor this whiteness (of false dawn) till (the true) dawn appears (or he said) till the dawn breaks.