Sahih Al Bukhari Hadith Number 1916

Narrated `Adi bin Hatim: When the above verses were revealed: ‘Until the white thread appears to you, distinct from the black thread,’ I took two (hair) strings, one black and the other white, and kept them under my pillow and went on looking at them throughout the night but could not make anything out of it. So, the next morning I went to Allah’s Messenger (peace be upon him) and told him the whole story. He explained to me, “That verse means the darkness of the night and the whiteness of the dawn.”

30  Fasting
Hadith number 1916
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.”)