كتاب الصيام
13  
The Book of Fasting
(8)
Chapter: Clarifying that fasting begins at dawn, and a person may eat and other than that until dawn begins; And clarifying the dawn which has to do with the rulings concerning the beginning of fasting and the beginning of the time for the Subh Prayer, and other than that, which is the Second Dawn, which is called the True Dawn. The First Dawn, which is the False Dawn, has nothing to do with the rulings
(8)
بَاب بَيَانِ أَنَّ الدُّخُولَ فِي الصَّوْمِ يَحْصُلُ بِطُلُوعِ الْفَجْرِ وَأَنَّ لَهُ الْأَكْلَ وَغَيْرَهُ حَتَّى يَطْلُعَ الْفَجْرُ وَبَيَانِ صِفَةِ الْفَجْرِ الَّذِي تَتَعَلَّقُ بِهِ الْأَحْكَامُ مِنْ الدُّخُولِ فِي الصَّوْمِ وَدُخُولِ وَقْتِ صَلَاةِ الصُّبْحِ وَغَيْرِ ذَلِكَ
Sahih Muslim 1093 c

This hadith has been narrated on the authority of Sulaiman Taimi with the same chain of transmitters and, at the end, it was said that the first Adhan was meant to awaken those who were in slumber amongst them and in order to make them turn who stand in (prayer) among them (towards food at the commencement of the fast). Jarir (one of the narrators) said that the Messenger (ﷺ) did not say like this but he said like it (true dawn) that the streaks of (true dawn ) are horizontal and not vertical.

وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا مُعْتَمِرُ بْنُ سُلَيْمَانَ، ح وَحَدَّثَنَا إِسْحَاقُ بْنُ، إِبْرَاهِيمَ أَخْبَرَنَا جَرِيرٌ، وَالْمُعْتَمِرُ بْنُ سُلَيْمَانَ، كِلاَهُمَا عَنْ سُلَيْمَانَ التَّيْمِيِّ، بِهَذَا الإِسْنَادِ ‏.‏ وَانْتَهَى حَدِيثُ الْمُعْتَمِرِ عِنْدَ قَوْلِهِ ‏"‏ يُنَبِّهُ نَائِمَكُمْ وَيَرْجِعُ قَائِمَكُمْ ‏"‏ ‏.‏ وَقَالَ إِسْحَاقُ قَالَ جَرِيرٌ فِي حَدِيثِهِ ‏"‏ وَلَيْسَ أَنْ يَقُولَ هَكَذَا وَلَكِنْ يَقُولُ هَكَذَا ‏"‏ ‏.‏ يَعْنِي الْفَجْرَ هُوَ الْمُعْتَرِضُ وَلَيْسَ بِالْمُسْتَطِيلِ ‏.‏
Reference : Sahih Muslim 1093c
In-book reference : Book 13, Hadith 49
USC-MSA web (English) reference : Book 6, Hadith 2406
  (deprecated numbering scheme)