Sahih Muslim Book 6, Hadith Number 2404.

Chapter : The timing of fast begins with the dawn.

Ibn Mas’ud (Allah be pleased with him) reported Allah’s Messenger (may peace be upon him) as saying The Adhan of Bilal should not restrain anyone among you from eating Sahur (last meal before daybreak during the month of Ramadan) for he announces Adhan (or he calls) at (the fag end of) the night to make him turn who stands for prayer among you, and to awaken those who are sleeping among you. And he said: The dawn is not like it, as one says (and he lifted his hand) till he (dispersed his fingers) and said: It is like this.