From Sahih Muslim (Hadith 2533):
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 (ﷺ) said: Your pillow seems to be very large. For the word khait implies the blackness of the night and the whiteness of the dawn
Reference: Muslim 2533