(16) بَابُ قَوْلِ اللَّهِ تَعَالَى: {وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ مِنَ الْفَجْرِ ثُمَّ أَتِمُّوا الصِّيَامَ إِلَى اللَّيْلِ}
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."
Reference: Sahih al-Bukhari 1916
In-book reference: Book 30, Hadith 26
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 31, Hadith 140