Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / 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 pro- nounced at the conclusion of the Sahri and the commencement of the fast).