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: "Let not the adhan of Bilal (may Allah be pleased with him) deceive you about your suhur, nor the whiteness that spreads straight up along the horizon, until it spreads out like this." And Hammad showed how with his hand, meaning spreading widthwise.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1787