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 Messenger of Allah 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."  

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 ﷺ فَذَكَرْتُ لَهُ ذَلِكَ فَقَالَ إِنَّمَا ذَلِكَ سَوَادُ اللَّيْلِ وَبَيَاضُ النَّهَارِ  

When the verse "Until the white thread of dawn appear to you distinct from its black thread" was revealed, I took a white rope and a black rope, and placed them beneath my pillow ; and then I looked at them, byt they were not clear to me. So I mentioned it to the Messenger of Allah ﷺ. He laughed and said: Your pillow is so broad and lengthy ; that is (i.e. means) night and day. The version of the narrator 'Uthman has: That is the blackness of night and whiteness of day.  

لَمَّا نَزَلَتْ هَذِهِ الآيَةُ { حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ } قَالَ أَخَذْتُ عِقَالاً أَبْيَضَ وَعِقَالاً أَسْوَدَ فَوَضَعْتُهُمَا تَحْتَ وِسَادَتِي فَنَظَرْتُ فَلَمْ أَتَبَيَّنْ فَذَكَرْتُ ذَلِكَ لِرَسُولِ اللَّهِ ﷺ فَضَحِكَ فَقَالَ إِنَّ وِسَادَكَ لَعَرِيضٌ طَوِيلٌ إِنَّمَا هُوَ اللَّيْلُ وَالنَّهَارُ قَالَ عُثْمَانُ إِنَّمَا هُوَ سَوَادُ اللَّيْلِ وَبَيَاضُ النَّهَارِ  

"When 'Until the white (light) thread of dawn appears distinct to you from the black thread (of night)' was revealed, the Prophet ﷺ said to me: 'That only refers to the whiteness of the day from the blackness of the night.'"  

لَمَّا نَزَلَتْ ( حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ مِنَ الْفَجْرِ ) قَالَ لِي النَّبِيُّ ﷺ إِنَّمَا ذَاكَ بَيَاضُ النَّهَارِ مِنْ سَوَادِ اللَّيْلِ  

[Machine] "When this verse was revealed: 'until the white thread of dawn becomes distinct from the black thread [of night],' I took a black thread and a white thread and placed them under my pillow. Then I looked but I could not distinguish between them, so I mentioned that to the Messenger of Allah ﷺ and he laughed and said, 'Your pillow must be very wide and long. This refers to the night and the day.'"  

لَمَّا نَزَلَتْ هَذِهِ الْآيَةُ {حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الْأَبْيَضُ مِنَ الْخَيْطِ الْأَسْوَدِ مِنَ الْفَجْرِ} أَخَذْتُ عِقَالًا أَسْوَدَ وَعِقَالًا أَبْيَضَ فَوَضَعْتُهُمَا تَحْتَ وِسَادَتِي فَنَظَرْتُ فَلَمْ أَتَبَيَّنْ فَذَكَرْتُ ذَلِكَ لِرَسُولِ اللهِ ﷺ فَضَحِكَ وَقَالَ «إِنَّ وِسَادَكَ إِذًا لَعَرِيضٌ طَوِيلٌ إِنَّمَا هَذَا اللَّيْلُ وَالنَّهَارُ»