The Messenger of Allah ﷺ said, "Let none of you be kept from his pre-dawn meal by Bilal's call to prayer, for he calls out to wake the sleeper among you and to send the one praying back to rest. The break of dawn is not when the light appears like this," and he gestured upward, "but rather like this," and he gestured horizontally across the horizon of the sky.
حدثنا يحيى بن حكيم حدثنا يحيى بن سعيد وابن أبي عدي عن سليمان التيمي عن أبي عثمان النهدي
أن رسول الله ﷺ قال لا يمنعن أحدكم أذان بلال من سحوره فإنه يؤذن لينتبه نائمكم وليرجع قائمكم وليس الفجر أن يقول هكذا ولكن هكذا يعترض في أفق السماء
Bukhari transmitted it in the Book of Adhan, chapter "The Adhan Before Fajr," hadith no. 621, and Muslim in the Book of Fasting, chapter "Explaining That Entering the Fast..." etc., hadith no. 1093, from the hadith of Sulayman Taymi through the same chain.