Adi bin Hatim reported,

When the verse was revealed, "until the white thread becomes distinct to you from the black thread of dawn" [Baqarah 2:187], Adi bin Hatim said to him, "Messenger of Allah, I place two cords beneath my pillow, one white and one black, so I can tell night from day."The Messenger of Allah ﷺ replied, "Your pillow must be very wide indeed. It refers only to the blackness of night and the whiteness of day."

Muslim 2533

Sahih

حدثنا أبو بكر بن أبي شيبة حدثنا عبد الله بن إدريس عن حصين عن الشعبي

عن عدي بن حاتم رضي الله عنه قال

لما نزلت {حتى يتبين لكم الخيط الأبيض من الخيط الأسود من الفجر} قال له عدي بن حاتم يا رسول الله إني أجعل تحت وسادتي عقالين عقالا أبيض وعقالا أسود أعرف الليل من النهار فقال رسول الله ﷺ إن وسادتك لعريض إنما هو سواد الليل وبياض النهار

مسلم 2533صحيح
Contents
Reference
Commentary
Ayat
Collection
Sahih Muslim
Book
Fasting
Chapter
Clarification that entering into fasting occurs at the rising of Fajr, and that one may eat and otherwise until Fajr rises, and clarification of the description of Fajr to which rulings are attached regarding entering into fasting, the beginning of the time for the morning prayer, and other matters
Numbering
Darussalam 2533
Muslim 2533