"Bilal calls the Adhan during the night, so eat and drink until Ibn Umm Maktum calls (the Adhan)" (Using translation from Nasāʾī 637)  

أَنَّ رَسُولَ اللهِ ﷺ قَالَ «إِنَّ بِلَالًا يُؤَذِّنُ بِلَيْلٍ فَكُلُوا وَاشْرَبُوا حَتَّى يُنَادِيَ ابْنُ أُمِّ مَكْتُومٍ»  

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). (Using translation from Muslim 1092a)  

أَنَّ النَّبِيَّ ﷺ قَالَ «إِنَّ بِلَالًا يُؤَذِّنُ بِلَيْلٍ فَكُلُوا وَاشْرَبُوا حَتَّى تَسْمَعُوا تَأْذِينَ ابْنِ أُمِّ مَكْتُومٍ»