(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
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.
Reference: Sahih Muslim 1093 a
In-book reference: Book 13, Hadith 47
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2404