Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) said: "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 the dawn' had not yet been revealed, some people, when they wanted to fast, would tie a black thread and a white thread to their feet, and would keep eating and drinking until one of the two became distinct to them. Then after that Allah Almighty revealed the words 'of the dawn' [Surah al-Baqarah: 187], and they knew that it meant (the distinction between) night and day."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5770