Musnad Ahmad 774:2

Hushaym narrated to us, Husayn informed us, from al-Sha'bi, 'Adi ibn Hatim informed us, he said: When this verse was revealed: "And eat and drink until the white thread becomes distinct to you from the black thread" [Al-Baqarah: 187], I took two ropes, one of them black and the other white, and I placed them under my pillow. He said: Then I began to look at them, but the black was not distinct to me from the white, nor the white from the black. When morning came, I went to the Messenger of Allah, peace and blessings of Allah be upon him, and informed him of what I had done. He said: "Your pillow is indeed wide if [it contains both]. It is only the whiteness of the day from the blackness of the night."