"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