Sunan an-Nasa'i 641
It was narrated from Ibn Mas'ud that the Prophet (S.A.W) said:
"Bilal calls the Adhan during the night to wake those who are sleeping and so that those who are praying Qiyam can return.[1] Not to say it is like this." The break of dawn is not like this. [2]

[1] Meaning to finish. Ash-Shawkani said: "To return to sleeping or return to sitting from praying" Nail Al-Awtar.

[2] Indicating with an up and down motion. The true dawn is from right to left.
أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ أَنْبَأَنَا الْمُعْتَمِرُ بْنُ سُلَيْمَانَ، عَنْ أَبِيهِ، عَنْ أَبِي عُثْمَانَ، عَنِ ابْنِ مَسْعُودٍ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏
"‏ إِنَّ بِلاَلاً يُؤَذِّنُ بِلَيْلٍ لِيُوقِظَ نَائِمَكُمْ وَلِيَرْجِعَ قَائِمَكُمْ وَلَيْسَ أَنْ يَقُولَ هَكَذَا ‏"‏ ‏.‏ يَعْنِي فِي الصُّبْحِ ‏.‏