Imam Bukhari narrates: Ahmad bin Yunus narrated to us, he said: Zuhair narrated to us, he said: Sulaiman al-Taimi narrated to us from Abu Uthman al-Nahdi from Abdullah bin Mas'ud, may Allāh be pleased with him, from the Prophet ﷺ, he said:
Let not the Adhan of Bilal prevent any one of you from your suhur meal, because he gives the Adhan at night, so that the one who is standing in prayer may return, and the one who is asleep may awaken. He said, moving his fingers from up to down: Fajr, or morning, is not like this, until he said: It is like this. Zuhair placed one of his finger over the other and drew it from right to left to show it.
حَدَّثَنَا أَحْمَدُ بْنُ يُونُسَ، قَالَ: حَدَّثَنَا زُهَيْرٌ، قَالَ: حَدَّثَنَا سُلَيْمَانُ التَّيْمِيُّ، عَنْ أَبِي عُثْمَانَ النَّهْدِيِّ، عَنْ عَبْدِ اللَّهِ بْنِ مَسْعُودٍ، عَنِ النَّبِيِّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قَالَ: «لاَ يَمْنَعَنَّ أَحَدَكُمْ - أَوْ أَحَدًا مِنْكُمْ - أَذَانُ بِلاَلٍ مِنْ سَحُورِهِ، فَإِنَّهُ يُؤَذِّنُ - أَوْ يُنَادِي بِلَيْلٍ - لِيَرْجِعَ قَائِمَكُمْ، وَلِيُنَبِّهَ نَائِمَكُمْ، وَلَيْسَ أَنْ يَقُولَ الفَجْرُ - أَوِ الصُّبْحُ -» وَقَالَ بِأَصَابِعِهِ وَرَفَعَهَا إِلَى فَوْقُ وَطَأْطَأَ إِلَى أَسْفَلُ حَتَّى يَقُولَ هَكَذَا وَقَالَ زُهَيْرٌ: «بِسَبَّابَتَيْهِ إِحْدَاهُمَا فَوْقَ الأُخْرَى، ثُمَّ مَدَّهَا عَنْ يَمِينِهِ وَشِمَالِهِ»
Reference: Sahih Al Bukhari 621