Sayyiduna Samurah bin Jundub (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "Do not let the adhan of Bilal deceive you about your sahur, nor the whiteness of the horizon that is elongated like this, until it spreads out like this." Hammad demonstrated with his hand, meaning: spread across.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8001