From Sahih Muslim (Hadith 2534):
Sahl b. Sa'd said that when this verse was revealed:" Eat and drink till the white streak is distinct from the dark streak," a person would take hold of a white thread and a black thread and keep on eating till he could find them distinct (in the light of the dawn). It was then that Allah, the Majestic and Great, reveiled (the words) min al-fajr (from the dawn), and then it became clear (that the word khait refers to the streak of light in the dawn)
Reference: Muslim 2534