The Messenger of Allah ﷺ said, "Do not let any of you be prevented from his pre-dawn meal by Bilal's call to prayer, for he calls out," or he said, "he announces, so that the one praying among you may return and the one sleeping among you may wake up. The dawn is not when it appears like this." Musaddad said, Yahya brought his hands together until it looks like this, and Yahya extended his two index fingers.
حدثنا مسدد حدثنا يحيى عن التيمي ح و حدثنا أحمد بن يونس حدثنا زهير حدثنا سليمان التيمي عن أبي عثمان
قال رسول الله ﷺ لا يمنعن أحدكم أذان بلال من سحوره فإنه يؤذن أو قال ينادي ليرجع قائمكم وينتبه نائمكم وليس الفجر أن يقول هكذا قال مسدد وجمع يحيى كفيه حتى يقول هكذا ومد يحيى بأصبعيه السبابتين
Bin Yunus.Sourced by: Bukhari narrated it in Adhan, chapter on the adhan before Fajr, hadith no. 621, from... etc, hadith no. 1093, and Muslim, in Sawm, chapter clarifying that entering the fast occurs with the rising of Fajr, through the hadith of Sulayman Taymi.