The Messenger of Allah ﷺ was asked about the time of the dawn prayer. He instructed Bilal to call the iqamah for the prayer when dawn broke, then the next day he delayed it until full daylight appeared. Then he said, "Where is the one who asked about the time of the morning prayer? What lies between these two is the time."
حدثنا إسماعيل أخبرنا حميد الطويل
سئل رسول الله ﷺ عن وقت صلاة الصبح قال فأمر بلالا حين طلع الفجر فأقام الصلاة ثم أسفر من الغد حتى أسفر ثم قال أين السائل عن وقت صلاة الغداة ما بين هاتين أو قال هذين وقت
Its isnad is sahih. The hadith was narrated by Muslim, 1/429, no. 614, in the Book of Mosques, chapter on the times of the prayers, and Abu Dawud, 2/108, no. 395, similarly, and Nasai, 1/171, no. 544, in the Book of Times, chapter on the beginning of the time of dawn prayer, and Tirmidhi, 1/278, no. 149, similarly to Muslim, and he said, hasan sahih, and Ibn Majah, 1/219, no. 667, similarly to them.