Sahih Bukhari Volume 003, Book 031, Hadith Number 140.

Narated By '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 Apostle and told him the whole story. He explained to me, "That verse means the darkness of the night and the whiteness of the dawn."