Sayyiduna Adi bin Hatim (may Allah be pleased with him) says: When this verse was revealed: "And eat and drink until the white thread becomes distinct to you from the black thread of dawn" [Surah al-Baqarah: 187], I took two threads, one white and one black, and put them under my pillow. Then I would get up at night and look, but they did not become distinct to me. In the morning I went to the Prophet (peace and blessings be upon him) and told him what I had done. He (peace and blessings be upon him) smiled and said: "Your pillow is very wide! It only means the whiteness of the day becoming distinct from the darkness of the night."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7999