Sayyiduna Samurah bin Jundub (may Allah be pleased with him) said: the Messenger of Allah (peace and blessings be upon him) said: "Do not let the adhan of Bilal deceive you, nor this blackness (darkness), until the dawn breaks forth like this." And he (peace and blessings be upon him) gestured with his hand horizontally.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6951