It is narrated from Sayyiduna Samurah bin Jundub (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Do not let the call of Bilal deceive you, nor the whiteness that is seen at the top of the pre-dawn sky (that is, do not let the false dawn deceive you)."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6950