Sahih Muslim Book 6, Hadith Number 2396.

Chapter : The timing of fast begins with the dawn.

‘Adi b. Hatim (Allah be pleased with him) reported that when (this verse) was revealed: “Until the white streak of the dawn becomes distinct from the dark streak” (ii. 187) Adi b. Hatim said: Messenger of Allah, verily I keep underneath my pillow two strings, one white and the other black, by which I distinguish night from dawn. Upon this the Messenger of Allah (may peace be upon him) said: Your pillow seems to be very large. For the word khait implies the blackness of the night and the whiteness of the dawn.