"I asked the Messenger of Allah ﷺ about the fast, he said: 'Until the white (light) thread of dawn appears distinct to you from the black thread (of night)'" - he said: "So I took two ropes, one white and the other black to look at them. So the Messenger of Allah ﷺ said to me" - it was something that Sufyan (a sub narrator) did not remember - so he said: "It is only the night and the day."  

سَأَلْتُ رَسُولَ اللَّهِ ﷺ عَنِ الصَّوْمِ فَقَالَ ( حَتََّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ ) قَالَ فَأَخَذْتُ عِقَالَيْنِ أَحَدُهُمَا أَبْيَضُ وَالآخَرُ أَسْوَدُ فَجَعَلْتُ أَنْظُرُ إِلَيْهِمَا فَقَالَ لِي رَسُولُ اللَّهِ ﷺ شَيْئًا لَمْ يَحْفَظْهُ سُفْيَانُ قَالَ إِنَّمَا هُوَ اللَّيْلُ وَالنَّهَارُ