I heard the Messenger of Allah (ﷺ) as saying: Bilal announces Adhan during the night, so you eat and drink, till you hear the Adhan of Ibn Umm Maktum.
Sahih Muslim Book 13, Hadith 43
← Previous Hadith 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).

Sahih Muslim Book 13, Hadith 42 Next Hadith → 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).

Sahih Muslim Book 13, Hadith 44