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 ‏.‏

It was narrated from Ibn Mas'ud that the Prophet (PBUH) 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. Translation Not Available

Book Ref: Sunan An Nisai Book 7 Hadith 648
Web Ref:  Sunan An Nisai Vol 1 Book 7 Hadith 642