He asked the Messenger of Allah ﷺ about the meaning of Allah's saying, "until the white thread becomes distinct to you from the black thread" [Baqarah 2:187]. He replied, "It refers to the darkness of night and the brightness of day."
أخبرنا علي بن حجر قال حدثنا جرير عن مطرف عن الشعبي
أنه سأل رسول الله ﷺ عن قوله تعالى {حتى يتبين لكم الخيط الأبيض من الخيط الأسود} قال هو سواد الليل وبياض النهار
Bukhari narrated it in tafsir, chapter, "Eat and drink until it becomes clear to you..." etc., hadith no. 4510, from the hadith of Jareer bin Abdul-Hameed. And Muslim narrated it in the Book of Fasting, chapter clarifying that entering into the fast occurs with the rising of Fajr... etc., hadith no. 1090, from the hadith of Shaybi, through him. And it is in Kubra, hadith no. 2479.