Sahih al-Bukhari 7247
Narrated Ibn Mas`ud:
Allah's Messenger (ﷺ) said, "The (call for prayer) Adhan of Bilal should not stop anyone of you from taking 
his Suhur for he pronounces the Adhan in order that whoever among you is praying the night prayer, 
may return (to eat his Suhur) and whoever among you is sleeping, may get up, for it is not yet dawn 
(when it is like this)." (Yahya, the sub-narrator stretched his two index fingers side ways).
حَدَّثَنَا مُسَدَّدٌ، عَنْ يَحْيَى، عَنِ التَّيْمِيِّ، عَنْ أَبِي عُثْمَانَ، عَنِ ابْنِ مَسْعُودٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏
"‏ لاَ يَمْنَعَنَّ أَحَدَكُمْ أَذَانُ بِلاَلٍ مِنْ سَحُورِهِ، فَإِنَّهُ يُؤَذِّنُ ـ أَوْ قَالَ يُنَادِي ـ لِيَرْجِعَ قَائِمَكُمْ، وَيُنَبِّهَ نَائِمَكُمْ، وَلَيْسَ الْفَجْرُ أَنْ يَقُولَ هَكَذَا ـ وَجَمَعَ يَحْيَى كَفَّيْهِ ـ حَتَّى يَقُولَ هَكَذَا ‏"‏‏.‏ وَمَدَّ يَحْيَى إِصْبَعَيْهِ السَّبَّابَتَيْنِ‏.‏