Sahih Al Bukhari Hadith Number 1917

Narrated Sahl bin Saud: When the following verses were revealed: ‘Eat and drink until the white thread appears to you, distinct from the black thread’ and of dawn was not revealed, some people who intended to fast, tied black and white threads to their legs and went on eating till they differentiated between the two. Allah then revealed the words, ‘of dawn’, and it became clear that meant night and day.

30  Fasting
Hadith number 1917
Volume no 3
Chapter: 16 (The Statement of Allah Taa’la: “And eat and drink until the white thread of dawn appears to you distinct from the black.”)