Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2398
Sahl b. Sa'd (Allah be pleased with him) said: When this verse was reveal- ed." Eat and drink till the white streak becomes distinct from the dark streak for you," the person who decided to observe fast tied on one of his feet a black thread and on the other a white thread. And he went on eating and drinking till he could distinguish (between their colour) on seeing them. It was after this that Allah reveal- ed (the words): min al-fajr. And they (the Muslims) came to know that (the word khait) refers to the night and day.