Sahih Bukhari Volume 006, Book 060, Hadith Number 037.
Narrated By 'Adi bin Hatim : I said, "O Allah's Apostle! What is the meaning of the white thread distinct from the black thread? Are these two threads?" He said, "You are not intelligent if you watch the two threads." He then added, "No, it is the darkness of the night and the whiteness of the day."