Sahih Muslim Book 6, Hadith Number 2398.

Chapter : The timing of fast begins with the dawn.

Sahl b. Sa’d (Allah be pleased with him) said: When this verse was revealed. “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 revealed (the words): min al-fajr. And they (the Muslims) came to know that (the word khait) refers to the night and day.