...And eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of the night).. . (V.2:187)

`Adi took a white rope (or thread) and a black one, and when some part of the night had passed, he looked at them but he could not distinguish one from the other. The next morning he said, "O Allah's Apostle! I put (a white thread and a black thread) underneath my pillow." The Prophet (ﷺ) said, "Then your pillow is too wide if the white thread (of dawn) and the black thread (of the night) are underneath your pillow! "

I said, "O Allah's Messenger (ﷺ)! 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.''

from the black thread." was revealed, but: '... of dawn' was not revealed (along with it) so some men, when intending to fast, used to tie their legs, one with white thread and the other with black thread and would keep on eating till they could distinguish one thread from the other. Then Allah revealed'... of dawn,' whereupon they understood that meant the night and the day.