Sayyiduna Adi bin Hatim (may Allah be pleased with him) said: When this verse was revealed: "And eat and drink until the white thread of the dawn becomes distinct to you from the black thread" [Surah al-Baqarah: 187], I took two cords, one black and the other white, and placed them under my pillow. At night I lifted them up and looked, but I could not distinguish the white from the black. I mentioned this to the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) said: "O Ibn Hatim! Then your pillow must be very wide! It only means: eat and drink until the whiteness of the day becomes distinct to you from the blackness of the night."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14251