The Prophet ﷺ said, "Bilal calls the adhan at night to wake those of you who are sleeping and to send back those of you who are praying. The dawn is not when it appears like this," and he gestured with his palm, "but the dawn is when it appears like this," and he pointed with his two index fingers.
أخبرنا عمرو بن علي قال حدثنا يحيى قال حدثنا التيمي عن أبي عثمان
عن النبي ﷺ قال إن بلالا يؤذن بليل لينبه نائمكم ويرجع قائمكم وليس الفجر أن يقول هكذا وأشار بكفه ولكن الفجر أن يقول هكذا وأشار بالسبابتين
[Sahih] mentioned previously, no. 642, and it is in Kubra, no. 2480 * Yahya is Qattan, Taymi is Sulayman bin Tarkhan, Abu Uthman is Nahdi.