Bukhari 4511

Narrated Sahl bin Sa`d The Verse "And eat and drink until the white thread appears to you distinct from the black thread." was revealed, but: '... of dawn' was not revealed (along with it). So some men, when intending to fast, used to tie their legs, one with white thread and the other with black thread and would keep on eating till they could distinguish one thread from the other. Then Allah revealed' ... of dawn,' whereupon they understood that meant the night and the day