Narrated `Adi bin Hatim:
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.''
USC-MSA web (English) reference: Vol. 6, Book 60, Hadith 37
Arabic reference: Book 65, Hadith 4510