Sahih Bukhari, Chapter: Fasting

When the 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."