Sahih Bukhari Volume 1, Book 10, Hadith 621

حَدَّثَنَا أَحْمَدُ بْنُ يُونُسَ، قَالَ حَدَّثَنَا زُهَيْرٌ، قَالَ حَدَّثَنَا سُلَيْمَانُ التَّيْمِيُّ، عَنْ أَبِي عُثْمَانَ النَّهْدِيِّ، عَنْ عَبْدِ اللَّهِ بْنِ مَسْعُودٍ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏"‏ لاَ يَمْنَعَنَّ أَحَدَكُمْ ـ أَوْ أَحَدًا مِنْكُمْ ـ أَذَانُ بِلاَلٍ مِنْ سَحُورِهِ، فَإِنَّهُ يُؤَذِّنُ ـ أَوْ يُنَادِي ـ بِلَيْلٍ، لِيَرْجِعَ قَائِمَكُمْ وَلِيُنَبِّهَ نَائِمَكُمْ، وَلَيْسَ أَنْ يَقُولَ الْفَجْرُ أَوِ الصُّبْحُ ‏"‏‏.‏ وَقَالَ بِأَصَابِعِهِ وَرَفَعَهَا إِلَى فَوْقُ وَطَأْطَأَ إِلَى أَسْفَلُ حَتَّى يَقُولَ هَكَذَا‏.‏ وَقَالَ زُهَيْرٌ بِسَبَّابَتَيْهِ إِحْدَاهُمَا فَوْقَ الأُخْرَى ثُمَّ مَدَّهَا عَنْ يَمِينِهِ وَشِمَالِهِ‏

Narrated By 'Abdullah bin Mas'ud : The Prophet said, "The Adhan pronounced by Bilal should not stop you from taking Suhur, for he pronounces the Adhan at night, so that the one offering the late night prayer (Tahajjud) from among you might hurry up and the sleeping from among you might wake up. It does not mean that dawn or morning has started." Then he (the Prophet) pointed with his fingers and raised them up (towards the sky) and then lowered them (towards the earth) like this (Ibn Mas'ud imitated the gesture of the Prophet). Az-Zuhri gestured with his two index fingers which he put on each other and then stretched them to the right and left. These gestures illustrate the way real dawn appears. It spreads left and right horizontally. The dawn that appears in the high sky and lowers down is not the real dawn).