Sahih Muslim Book 6, Hadith Number 2399.

‘Abdullah b. Mas’ud (Allah be pleased with him) reported that the Messenger of Allah (may peace be upon him) had said: Bilal would pronounce Adhan (at the fag end of the night in order to inform the people about the time of the Sahri). So you eat and drink till you hear the Adhan of Ibn Umm Maktum (which was pronounced at the conclusion of the Sahri and the commencement of the fast).