The verse was revealed, "And eat and drink until the white thread becomes distinct to you from the black thread" [Baqarah 2:187], and the words "of the dawn" had not yet been revealed. So when men intended to fast, each of them would tie a white thread and a black thread to his legs, and he would keep eating until he could clearly see the two of them. Then Allah revealed afterward "of the dawn" [Baqarah 2:187], and they understood that what was meant was only the separation of night from day.
حدثنا ابن أبي مريم حدثنا أبو غسان محمد بن مطرف حدثني أبو حازم
وأنزلت {وكلوا واشربوا حتى يتبين لكم الخيط الأبيض من الخيط الأسود} ولم ينزل {من الفجر} وكان رجال إذا أرادوا الصوم ربط أحدهم في رجليه الخيط الأبيض والخيط الأسود ولا يزال يأكل حتى يتبين له رؤيتهما فأنزل الله بعده {من الفجر} فعلموا أنما يعني الليل من النهار