Musnad Ahmad 774:7

Yahya narrated to us, from Mujalid, who said: 'Amir informed me, and 'Adi ibn Hatim narrated to me, saying: The Messenger of Allah (peace and blessings of Allah be upon him) taught me prayer and fasting. He said: "Pray such and such, and fast. When the sun sets, eat and drink until the white thread becomes distinct to you from the black thread. And fast thirty days, unless you see the crescent before that." So I took two threads of black and white hair and I would look at them, but it did not become distinct to me. I mentioned that to the Messenger of Allah (peace and blessings of Allah be upon him), and he laughed and said: "O son of Hatim, that is only the whiteness of the day from the blackness of the night."