I asked the Messenger of Allah ﷺ about fasting, and he recited, "Until the white thread becomes distinct to you from the black thread" [Baqarah 2:187].So I took two cords, one white and the other black, and kept looking at them. The Messenger of Allah ﷺ then said something to me which Sufyan did not memorize. He said, "It refers only to the night and the day."
حدثنا ابن أبي عمر حدثنا سفيان عن مجالد عن الشعبي
سألت رسول الله ﷺ عن الصوم فقال {حتى يتبين لكم الخيط الأبيض من الخيط الأسود} قال فأخذت عقالين أحدهما أبيض والآخر أسود فجعلت أنظر إليهما فقال لي رسول الله ﷺ شيئا لم يحفظه سفيان قال إنما هو الليل والنهار
Sourcing, [sahih]. Agreed upon by Bukhari and Muslim. Bukhari related it in the Book of Expeditions, Chapter on the Expedition of Hudaybiya, no. 4191, from the hadith of Hushaym, from Abu Bishr. Muslim also related it from Abu Bishr, and Muslim, no. 1201, related it from the hadith of Mujahid, through the same chain.The hadith of Abdullah bin Maqil was related by Bukhari, no. 1816, and by Muslim, no. 1201/85, from the hadith of Abdul-Rahman bin Asbahani, from him.