Narrated 'Abd Allah b. Mas'ud: The Messenger of Allah (ﷺ) as saying: The summons (adhan) of Bilal should not restrain one of you from taking a meal shortly before dawn, for he utters adhan or calls (for prayer) so that the man at prayer may return, and the man asleep may get up. Dawn is not (the whiteness) which indicates thus (in perpendicular) - the narrator Musaddad said: Yahya joined his palms (indicating the spread of whiteness vertically - until it indicates thus - and Yahya spread out two ring-fingers of his (demonstrating the spread of whiteness horizontally)l
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَحْيَى، عَنِ التَّيْمِيِّ، ح وَحَدَّثَنَا أَحْمَدُ بْنُ يُونُسَ، حَدَّثَنَا زُهَيْرٌ، حَدَّثَنَا سُلَيْمَانُ التَّيْمِيُّ، عَنْ أَبِي عُثْمَانَ، عَنْ عَبْدِ اللَّهِ بْنِ مَسْعُودٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " لاَ يَمْنَعَنَّ أَحَدَكُمْ أَذَانُ بِلاَلٍ مِنْ سَحُورِهِ فَإِنَّهُ يُؤَذِّنُ - أَوْ قَالَ يُنَادِي - لِيَرْجِعَ قَائِمُكُمْ وَيَنْتَبِهَ نَائِمُكُمْ وَلَيْسَ الْفَجْرُ أَنْ يَقُولَ هَكَذَا " . قَالَ مُسَدَّدٌ وَجَمَعَ يَحْيَى كَفَّيْهِ حَتَّى يَقُولَ هَكَذَا وَمَدَّ يَحْيَى بِأُصْبَعَيْهِ السَّبَّابَتَيْنِ .
sunnah.com