Sayyiduna Adi bin Hatim (may Allah be pleased with him) said: I said: O Messenger of Allah! What is meant by the white thread from the black thread? Are they really two threads? He (peace and blessings be upon him) said: "Your nape must be very broad indeed if you have (really) seen the two threads!" Then he said: "No, rather it means the blackness of the night and the whiteness of the day."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14250