Adi bin Hatim reported,

I asked, "Messenger of Allah, what is meant by 'the white thread from the black thread' [Baqarah 2:187]? Are they actual threads?"He replied, "Your neck must be quite broad if you can see the two threads!" Then he said, "No, rather it is the blackness of the night and the whiteness of the day."

Bukhari 4510

Sahih

حدثنا قتيبة بن سعيد حدثنا جرير عن مطرف عن الشعبي

عن عدي بن حاتم رضي الله عنه قال

قلت يا رسول الله ما {الخيط الأبيض من الخيط الأسود} أهما الخيطان قال إنك لعريض القفا إن أبصرت الخيطين ثم قال لا بل هو سواد الليل وبياض النهار

البخاري 4510صحيح
Contents
Reference
Commentary
Ayat
Collection
Sahih Bukhari
Book
Tafsir
Chapter
His saying, "And eat and drink until the white thread of dawn becomes distinct to you from the black thread of Fajr. Then complete the fast until night, and do not have relations with them while you are secluded in the mosques"
Numbering
Darussalam 4510
Bukhari 4510