Sahih al-Bukhari 4509
Sahih al-Bukhari · Book 65: Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh)) · "...And eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of the night).. ." (V.2:187)
Narrated Ash-Shu`bi:
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 " .
`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 (ﷺ) said, "Then your pillow is too wide if the white thread (of dawn) and the black thread (of the night) are underneath your pillow! "