Sahih Muslim Book 6, Hadith Number 2406.
Chapter : The timing of fast begins with the dawn.
This Hadith has been narrated on the authority of Sulaiman Taimi with the same chain of transmitters and, at the end, it was said that the first Adhan was meant to awaken those who were in slumber amongst them and in order to make them turn who stand in (prayer) among them (towards food at the commencement of the fast). Jarir (one of the narrators) said that the Messenger (may peace be upon him) did not say like this but he said like it (true dawn) that the streaks of (true dawn) are horizontal and not vertical.