Sayyiduna Adi bin Hatim (may Allah be pleased with him) said: When this verse was revealed: "Until the white thread of the dawn becomes distinct to you from the black thread" [Surah al-Baqarah: 187], I took a black cord and a white cord and placed them under my pillow, then kept looking, but the difference did not become clear to me. I mentioned this to the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) laughed and said: "Then your pillow must be very wide and long! It only means the night and the day."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14248