Sahih Muslim Book 6, Hadith Number 2401.
Chapter : The timing of fast begins with the dawn.
Ibn ‘Umar(Allah be pleased with both of them)reported that the Messenger of Allah (may peace be upon him) had two Mu’adhdhins, Bilal and son of Umm Maktum, the blind. The Messenger of Allah (may peace be upon him) said: Bilal announces Adhan at (the fag end of the) night (i.e. Sahri), so eat and drink till the son of Umm Maktum announces Adhan. And he (the narrator) said: And the (difference of time) between their (Adhans) was not more than this that one climbed down (from the minaret) and the other climbed up (to announce Adhan).