It is narrated from Sayyiduna Samurah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Do not let the adhan of Bilal, nor the elongated (vertical) dawn, prevent you from (eating) suhur; rather, the true dawn is the one that spreads across the horizon." These are the words of Hajjaj bin Nusayr.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6952