Sunan Ibn Majah Book 7, Hadith 1696

حَدَّثَنَا يَحْيَى بْنُ حَكِيمٍ، حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، وَابْنُ أَبِي عَدِيٍّ، عَنْ سُلَيْمَانَ التَّيْمِيِّ، عَنْ أَبِي عُثْمَانَ النَّهْدِيِّ، عَنْ عَبْدِ اللَّهِ بْنِ مَسْعُودٍ، أَنَّ رَسُولَ اللَّهِ ـ صلى الله عليه وسلم ـ قَالَ: ‏ "‏ لاَ يَمْنَعَنَّ أَحَدَكُمْ أَذَانُ بِلاَلٍ مِنْ سُحُورِهِ. فَإِنَّهُ يُؤَذِّنُ لِيَنْتَبِهَ نَائِمُكُمْ، وَلِيَرْجِعَ قَائِمُكُمْ. وَلَيْسَ الْفَجْرُ أَنْ يَقُولَ هَكَذَا. وَلَكِنْ هَكَذَا، يَعْتَرِضُ فِي أُفُقِ السَّمَاءِ ‏"‏ ‏.‏

It was narrated from ‘Abdullah bin Mas’ud that the Messenger of Allah (ﷺ) said:“The Adhan of Bilal should not prevent anyone of you from eating Suhur, for he gives the Adhan to alert those among you who are asleep, and so that anyone who is praying can prepare himself for fasting. The Fajr does not come in this manner, rather it comes in this manner, and it appears along the horizon.”