Sahih al-Bukhari 4510
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 `Adi bin Hatim:
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ ، حَدَّثَنَا جَرِيرٌ ، عَنْ مُطَرِّفٍ ، عَنِ الشَّعْبِيِّ ، عَنْ عَدِيِّ بْنِ حَاتِمٍ ـ رضى الله عنه ـ قَالَ قُلْتُ يَا رَسُولَ اللَّهِ مَا الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ أَهُمَا الْخَيْطَانِ قَالَ " إِنَّكَ لَعَرِيضُ الْقَفَا إِنْ أَبْصَرْتَ الْخَيْطَيْنِ ". ثُمَّ قَالَ " لاَ بَلْ هُوَ سَوَادُ اللَّيْلِ وَبَيَاضُ النَّهَارِ " .
I said, "O Allah's Messenger (ﷺ)! What is the meaning of the white thread distinct from the black thread? Are these two threads?" He said, "You are not intelligent if you watch the two threads." He then added, "No, it is the darkness of the night and the whiteness of the day.''