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 b

This hadith has been narrated by Sulaiman al-Taimi with the same chain of transmitters (but with a slight variation of words) that he (the Holy Prophet) said:

The dawn is not like it as it is said; he then gathered his fingers and lowered them. But he said, it is like this (and he placed the index finger upon the other one and spread his hand).
حَدَّثَنَا ابْنُ نُمَيْرٍ، حَدَّثَنَا أَبُو خَالِدٍ، - يَعْنِي الأَحْمَرَ - عَنْ سُلَيْمَانَ التَّيْمِيِّ، بِهَذَا الإِسْنَادِ غَيْرَ أَنَّهُ قَالَ ‏"‏ إِنَّ الْفَجْرَ لَيْسَ الَّذِي يَقُولُ هَكَذَا ‏"‏ ‏.‏ وَجَمَعَ أَصَابِعَهُ ثُمَّ نَكَسَهَا إِلَى الأَرْضِ ‏"‏ وَلَكِنِ الَّذِي يَقُولُ هَكَذَا ‏"‏ ‏.‏ وَوَضَعَ الْمُسَبِّحَةَ عَلَى الْمُسَبِّحَةِ وَمَدَّ يَدَيْهِ ‏.‏
Reference : Sahih Muslim 1093b
In-book reference : Book 13, Hadith 48
USC-MSA web (English) reference : Book 6, Hadith 2405
  (deprecated numbering scheme)