Sunan at-Tirmithiy كتاب الصوم باب ما جاء فى بيان الفجر

حَدَّثَنَا هَنَّادٌ وَيُوسُفُ بْنُ عِيسَى قَالاَ حَدَّثَنَا وَكِيعٌ عَنْ أَبِى هِلاَلٍ عَنْ سَوَادَةَ بْنِ حَنْظَلَةَ هُوَ الْقُشَيْرِىُّ عَنْ سَمُرَةَ بْنِ جُنْدَبٍ قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم لاَ يَمْنَعَنَّكُمْ مِنْ سُحُورِكُمْ أَذَانُ بِلاَلٍ وَلاَ الْفَجْرُ الْمُسْتَطِيلُ وَلَكِنِ الْفَجْرُ الْمُسْتَطِيرُ فِى الأُفُقِ. قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ حَسَنٌ.
The Prophet, peace and blessings of God be upon him said, "Indeed, neither Bilāl's (2) call to prayer (3) nor the vertical dawn (4)should deprive you of your pre-dawn meal (1); rather it is the dawn which spreads along the horizon (5) (that signals the start of fasting)." Footnotes: (1) Saḥūr: an optional, yet desirable, meal eaten before dawn by Muslims planning to fast that day. (2) Bilāl Ibn-Rabāḥ is one of the companions of the Prophet, peace and blessings of God be upon him, who is famous for calling for prayers in Madīnah. (3) The call to prayer (Athān) is a long call announcing the time of prayer. It is usually made up to twenty minutes before the commencement of prayer to give worshippers time to prepare for prayer; it is called even longer before the dawn prayer (Fajr), the first prescribed prayer of the day that takes place before sunrise. (4) The vertical dawn, also known as the false dawn, is indicated by rays that spread vertically in the sky. This occurs about twenty minutes before the true dawn. (5) The second dawn, also known as the true dawn, is indicated by a white line that spreads parallel to the horizon. The occurrence of the true dawn signals the start of the dawn prayer's time and, consequentially, the beginning of fasting. Click on ḥadīth to read more