Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.113B.
Section : Essential elements of fasting, abstaining from acts that break the fast.
This point is based on the Qur’anic verse: “Eat and drink until the white thread becomes distinct to you from the black thread of the dawn. Then strictly observe the fast until nightfall.”
This is also based on the following hadith: “When the verse ‘Eat and drink until the white thread becomes distinct to you…’ was revealed, I took a black thread and a white thread and placed them underneath my pillow. During the night I looked at them to see if I could distinguish between them. In the morning I went to the Messenger of Allah and mentioned that to him and he said: ‘It is the black of the night and the white of the day.'”