Sayyiduna Adi bin Hatim (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) taught me the prayer and fasting, and said: "Pray such-and-such, and pray such-and-such, and fast like this. When the sun sets, eat and drink until the white thread of the dawn becomes distinct to you from the black thread, and fast thirty days unless you see the new moon before that." So I took two threads of hair, black and white, and kept looking at them, but the difference did not become clear to me. I mentioned this to the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) laughed and said: "O Ibn Hatim! That only means the whiteness of the day from the blackness of the night."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14244