The Prophet ﷺ said, "The true dawn is not the elongated light stretching along the horizon, but rather the red light that spreads across it."
قال حدثنا موسى حدثنا محمد بن جابر عن عبد الله بن النعمان
أن النبي ﷺ قال ليس الفجر المستطيل في الأفق ولكنه المعترض الأحمر
Its isnad is hasan, and Abdullah bin Numan Hanafi Yaami, his hadith has also been accepted. This hadith was narrated by Muslim, 2/769, no. 1092, in the Book of Fasting, on the clarification of entering into the fast. And Tirmidhi, 3/76, no. 705, who said, hasan gharib. And Nasai, 4/148, no. 2171, and Ibn Majah, 1/541, no. 1696, and similarly Abu Dawud, 2/304, no. 2348.