Narrated `Adi bin Hatim: When the above verses were revealed: 'Until the white thread appears to you, distinct from the black thread,' I took two (hair) strings, one black and the other white, and kept them under my pillow and went on looking at them throughout the night but could not make anything out of it. So, the next morning I went to Allah's Messenger (ﷺ) and told him the whole story. He explained to me, "That verse means the darkness of the night and the whiteness of the dawn."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا حَجَّاجُ بْنُ مِنْهَالٍ، حَدَّثَنَا هُشَيْمٌ، قَالَ أَخْبَرَنِي حُصَيْنُ بْنُ عَبْدِ الرَّحْمَنِ، عَنِ الشَّعْبِيِّ، عَنْ عَدِيِّ بْنِ حَاتِمٍ ـ رضى الله عنه ـ قَالَ لَمَّا نَزَلَتْ {حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ} عَمَدْتُ إِلَى عِقَالٍ أَسْوَدَ وَإِلَى عِقَالٍ أَبْيَضَ، فَجَعَلْتُهُمَا تَحْتَ وِسَادَتِي، فَجَعَلْتُ أَنْظُرُ فِي اللَّيْلِ، فَلاَ يَسْتَبِينُ لِي، فَغَدَوْتُ عَلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فَذَكَرْتُ لَهُ ذَلِكَ فَقَالَ " إِنَّمَا ذَلِكَ سَوَادُ اللَّيْلِ وَبَيَاضُ النَّهَارِ ".
sunnah.com