Sahih Bukhari Volume 6, Book 60, Hadith Number 36.

Narrated By Ash-Sha’bi : ‘Adi took a white rope (or thread) and a black one, and when some part of the night had passed, he looked at them but he could not distinguish one from the other. The next morning he said, “O Allah’s Apostle! I put (a white thread and a black thread) underneath my pillow.” The Prophet said, “Then your pillow is too wide if the white thread (of dawn) and the black thread (of the night) are underneath your pillow!”