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 ‏.‏