Sayyiduna Adi bin Hatim (may Allah be pleased with him) says: When this verse was revealed: "And eat and drink until the white thread becomes distinct to you from the black thread of dawn" [Surah al-Baqarah: 187], I took two threads, one white and one black, and put them under my pillow. Then I would get up at night and look, but they did not become distinct to me. In the morning I went to the Prophet (peace and blessings be upon him) and told him what I had done. He (peace and blessings be upon him) smiled and said: "Your pillow is very wide! It only means the whiteness of the day becoming distinct from the darkness of the night."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7999
Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) says: When this verse was revealed: "And eat and drink until the white thread becomes distinct to you from the black thread" [Surah al-Baqarah: 187], and the words "of dawn" had not yet been revealed, then when people intended to fast, they would tie a white and a black thread to their leg, and they would go on eating and drinking until their colours became distinct. Then Allah Almighty revealed "of dawn", and they came to know that what was meant by it was the night and the day.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8000
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
Muhammad bin Abd al-Rahman bin Thawban (may Allah have mercy on him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "There are two dawns. As for the one that is like the tail of a wolf, it neither makes anything forbidden nor makes anything lawful. But the elongated one that spreads across the horizon makes the prayer lawful and makes food forbidden."
This is mursal (the Successor narrates directly from the Prophet), and it has also been narrated with a connected chain mentioning Jabir bin Abdullah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8002
Sayyiduna Ibn Abbas (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Dawn is of two kinds: the first neither makes food forbidden nor makes the prayer lawful; but the second dawn makes food forbidden and makes the prayer lawful."
Abu Ahmad al-Zubayri narrated it as a saying of the Prophet, while others narrated it from al-Thawri as mawquf (a statement of Ibn Abbas).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8003