Sahih Muslim Book 6, Hadith Number 2409.

Chapter : The timing of fast begins with the dawn.

Samura b. Jundub (Allah be pleased with him) reported Allah’s Messenger (may peace be upon him) as saying: The Adhan of Bilal may not mislead you with regard to your food at the commencement of the fast, nor the vertical (streaks) of whiteness in the horizon (for it is an indication of false dawn). You should stop eating (food) till (the whiteness) spreads like it. Hammad narrated it and with the gesture of his band he explained the horizontal position (of the streaks of light).