When this verse was revealed, "Until the white thread becomes distinct to you from the black thread" [Baqarah 2:187], he said, "I took a white cord and a black cord and placed them under my pillow. I looked at them but could not distinguish between them, so I mentioned that to the Messenger of Allah ﷺ."He laughed and said, "Your pillow must be very wide and long indeed. It refers only to the night and the day."Uthman said, "It refers only to the darkness of night and the brightness of day."
حدثنا مسدد حدثنا حصين بن نمير ح و حدثنا عثمان بن أبي شيبة حدثنا ابن إدريس المعنى عن حصين عن الشعبي
لما نزلت هذه الآية {حتى يتبين لكم الخيط الأبيض من الخيط الأسود} قال أخذت عقالا أبيض وعقالا أسود فوضعتهما تحت وسادتي فنظرت فلم أتبين فذكرت ذلك لرسول الله ﷺ فضحك فقال إن وسادك لعريض طويل إنما هو الليل والنهار و قال عثمان إنما هو سواد الليل وبياض النهار
Narrated by Muslim, Fasting, Chapter on the explanation that the fast begins with the appearance of Fajr, etc., no. 1090, from the hadith of Abdullah bin Idris.And Bukhari, Fasting, Chapter on Allah's saying, "Eat and drink until the white thread of dawn becomes clear to you from the black thread" [Baqarah 2:187], etc., no. 1916, from the hadith of Husayn bin Abdul-Rahman, through the same chain.