Narrated 'Adi bin Hatim:
"I asked the Messenger of Allah (ﷺ) about the fast, he said: 'Until the white (light) thread of dawn appears distinct to you from the black thread (of night)'" - he said: "So I took two ropes, one white and the other black to look at them. So the Messenger of Allah (ﷺ) said to me" - it was something that Sufyan (a sub narrator) did not remember - so he said: "It is only the night and the day."
English reference: Vol. 5, Book 44, Hadith 2971
Arabic reference: Book 47, Hadith 3235