Sahl bin Saad reported,

The verse was revealed, "Eat and drink until the white thread becomes distinct to you from the black thread" [Baqarah 2:187], and the words "of dawn" had not yet been revealed. So when men intended to fast, one of them would tie a white thread and a black thread to his leg, and he would keep eating until he could clearly see the two of them. Then Allah revealed afterward the words "of dawn" [Baqarah 2:187], and they understood that what was meant was night and day.

Azami 11964

Sahih

عن سهل بن سعد: قال

أنزلت: {وكلوا واشربوا حتى يتبين لكم الخيط الأبيض من الخيط الأسود} ولم ينزل: {من الفجر} فكان رجال إذا أرادوا الصوم ربط أحدهم في رجله الخيط الأبيض والخيط الأسود، ولم يزل يأكل حتى يتبين له رؤيتهما، فأنزل الله بعد: {من الفجر} فعلموا أنه إنما يعني الليل والنهار.

الأعظمي 11964صحيح
Contents
Reference
Commentary
Ayat
Collection
Jami al-Kamil
Book
Tafsir of the great Quran
Chapter
Virtues of Surah Baqarah
Section
His saying, "It has been made permissible for you the night preceding fasting to go to your wives. They are clothing for you and you are clothing for them. Allah knows that you used to deceive yourselves, so He turned to you in forgiveness and pardoned you. So now have relations with them and seek that which Allah has decreed for you. 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 the night. And do not have relations with them as long as you are staying for worship in the mosques. These are the limits of Allah, so do not approach them. Thus does Allah make clear His verses to the people."
Numbering
Jami al-Kamil 11964 (2nd edition)
Azami 11964