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 following verses were revealed: 'Eat and drink until the white thread appears to you, distinct from the black thread' and of dawn was not revealed, some people who intended to fast, tied black and white threads to their legs and went on eating till they differentiated between the two. Allah then revealed the words, 'of dawn', and it became clear that meant night and day.

أُنْزِلَتْ {وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ} وَلَمْ يَنْزِلْ مِنَ الْفَجْرِ فَكَانَ رِجَالٌ إِذَا أَرَادُوا الصَّوْمَ رَبَطَ أَحَدُهُمْ فِي رِجْلِهِ الْخَيْطَ الأَبْيَضَ وَالْخَيْطَ الأَسْوَدَ وَلَمْ يَزَلْ يَأْكُلُ حَتَّى يَتَبَيَّنَ لَهُ رُؤْيَتُهُمَا فَأَنْزَلَ اللَّهُ بَعْدُ {مِنَ الْفَجْرِ} فَعَلِمُوا أَنَّهُ إِنَّمَا يَعْنِي اللَّيْلَ وَالنَّهَارَ