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 a

Ibn Mas'ud (Allah be pleased with him) reported Allah's Messenger (ﷺ) as saying The Adhan of Bilal should not restrain anyone among you from eating Sahur (last meal before daybreak during the month of Ramadan) for he announces Adhan (or he calls) at (the fag end of) the night to make him turn who stands for prayer among you, and to awaken those who are sleeping among you. And he said:

The dawn is not like it, as one says (and he lifted his hand) till he (dispersed his fingers) and said: It is like this.
حَدَّثَنَا زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا إِسْمَاعِيلُ بْنُ إِبْرَاهِيمَ، عَنْ سُلَيْمَانَ التَّيْمِيِّ، عَنْ أَبِي، عُثْمَانَ عَنِ ابْنِ مَسْعُودٍ، - رضى الله عنه - قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ لاَ يَمْنَعَنَّ أَحَدًا مِنْكُمْ أَذَانُ بِلاَلٍ - أَوْ قَالَ نِدَاءُ بِلاَلٍ - مِنْ سَحُورِهِ فَإِنَّهُ يُؤَذِّنُ - أَوْ قَالَ يُنَادِي - بِلَيْلٍ لِيَرْجِعَ قَائِمَكُمْ وَيُوقِظَ نَائِمَكُمْ ‏"‏ ‏.‏ وَقَالَ ‏"‏ لَيْسَ أَنْ يَقُولَ هَكَذَا وَهَكَذَا - وَصَوَّبَ يَدَهُ وَرَفَعَهَا - حَتَّى يَقُولَ هَكَذَا ‏"‏ ‏.‏ وَفَرَّجَ بَيْنَ إِصْبَعَيْهِ ‏.‏
Reference : Sahih Muslim 1093a
In-book reference : Book 13, Hadith 47
USC-MSA web (English) reference : Book 6, Hadith 2404
  (deprecated numbering scheme)