Sunan an Nasai
2171
Sahih+2

Samurah said:"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

SahihAbu Ghuddah
SahihAl-Albani