It was narrated from Ibn Mas'ud that the Prophet (ﷺ) said: "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. '
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا عَمْرُو بْنُ عَلِيٍّ، قَالَ حَدَّثَنَا يَحْيَى، قَالَ حَدَّثَنَا التَّيْمِيُّ، عَنْ أَبِي عُثْمَانَ، عَنِ ابْنِ مَسْعُودٍ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " إِنَّ بِلاَلاً يُؤَذِّنُ بِلَيْلٍ لِيُنَبِّهَ نَائِمَكُمْ وَيُرْجِعَ قَائِمَكُمْ وَلَيْسَ الْفَجْرُ أَنْ يَقُولَ هَكَذَا " . وَأَشَارَ بِكَفِّهِ " وَلَكِنِ الْفَجْرُ أَنْ يَقُولَ هَكَذَا " . وَأَشَارَ بِالسَّبَّابَتَيْنِ .
sunnah.com