Shaddad, the freed slave of Iyad, narrates from Sayyiduna Bilal (may Allah be pleased with him). Bilal (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Do not give the call to prayer until you see the dawn like this," and he (peace and blessings be upon him) gestured with his hand, then spread it out.
Hadith Referenceالمعجم الكبير للطبراني / باب الباء / 1117