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