"Bilal calls the Adhan at night to awaken those of you who are asleep, and so that those who are praying Qiyam cam return. Dawn is not when the light appears like this" - and he gestured with his hand - "rather dawn is when it appears like this: - and he gestured with his two forefingers. ' (Using translation from Nasāʾī 2170)  

عَنِ النَّبِيِّ ﷺ قَالَ «إِنَّ بِلَالًا يُؤَذِّنُ بِلَيْلٍ لِيُنَبَّهَ نَائِمُكُمْ وَيَرْجِعَ قَائِمُكُمْ وَلَيْسَ الْفَجْرُ أَنْ يَقُولَ هَكَذَا وَأَشَارَ بِكَفِّهِ وَلَكِنِ الْفَجْرُ أَنْ يَقُولَ هَكَذَا وَأَشَارَ بِالسَّبَّابَتَيْنِ»  

"The Messenger of Allah said; 'DO not be confused by the Adhan of Bilal, or by this whiteness, until dawn appears like this" - meaning horizontally. (One of the narrators) Abu Dawud said: "And he spread out his hands gesturing to the right and left." (Using translation from Nasāʾī 2171)   

قَالَ رَسُولُ اللهِ ﷺ «لَا يَغُرَّنَّكُمْ أَذَانُ بِلَالٍ وَلَا هَذَا الْبَيَاضُ حَتَّى يَنْفَجِرَ الْفَجْرُ هَكَذَا وَهَكَذَا» يَعْنِي مُعْتَرِضًا قَالَ أَبُو دَاوُدَ فَبَسَطَ يَدَيْهِ يَمِينًا وَشِمَالًا مَادًّا يَدَيْهِ