It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Let not the adhan of Bilal prevent any of you from eating the pre-dawn meal (suhur), for he only calls the adhan so that your sleeping one may wake up and the one standing in prayer may return, and (the true) dawn appears like this (spreading across the horizon)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10512