When the verse "until the white thread becomes distinct to you from the black thread" [Baqarah 2:187] was revealed, I took a black cord and a white cord and placed them under my pillow. I kept looking at them during the night, but they were not clearly distinguishable to me. So I went in the morning to the Messenger of Allah ﷺ and mentioned that to him.He said, "That refers only to the darkness of the night and the brightness of the day."
لما نزلت: {حتى يتبين لكم الخيط الأبيض من الخيط الأسود} عمدت إلى عقال أسود، وإلى عقال أبيض فجعلتهما تحت وسادتي، فجعلت أنظر في الليل فلا يستبين لي، فغدوت على رسول الله ﷺ فذكرت ذلك له، فقال: إنما ذلك سواد الليل وبياض النهار.
Bukhari narrated it in the Book of Fasting (1916), and Muslim narrated it in the Book of Fasting (1090), both through the route of Husayn bin Abdul-Rahman, from Shaybi, from Adi bin Hatim, mentioning it as such. The wording given here is Bukhari's. Muslim's wording is similar, with the addition, "Your pillow must be very wide."Regarding his saying, "your pillow." The pillow (wisadah) is the cushion, which is what one places under the head when sleeping.Regarding his saying, "very wide." Qadi Iyad said, its meaning is that if you placed under your pillow the two threads that Allah, Most High, intended, namely the night and the day, then your pillow would rise above them and cover them, and in that case it would indeed be wide.