Sahih Bukhari Volume 6, Book 65, Hadith 4511

حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ حُصَيْنٍ، عَنِ الشَّعْبِيِّ، عَنْ عَدِيٍّ، قَالَ أَخَذَ عَدِيٌّ عِقَالاً أَبْيَضَ وَعِقَالاً أَسْوَدَ حَتَّى كَانَ بَعْضُ اللَّيْلِ نَظَرَ فَلَمْ يَسْتَبِينَا، فَلَمَّا أَصْبَحَ قَالَ يَا رَسُولَ اللَّهِ، جَعَلْتُ تَحْتَ وِسَادَتِي‏.‏ قَالَ ‏"‏ إِنَّ وِسَادَكَ إِذًا لَعَرِيضٌ أَنْ كَانَ الْخَيْطُ الأَبْيَضُ وَالأَسْوَدُ تَحْتَ وِسَادَتِكَ ‏"‏‏.‏

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!"