Mukhtasar Muslim (Abridged) 583 — About Allah’s saying: “Until

Sahl bin Sa’d (may Allah be pleased with him) says that when this verse was revealed: “Eat and drink until the white thread becomes distinct to you from the black thread” { البقرہ : 178 }, a man who intended to fast would tie two threads to his foot, one white and the other black, and would continue eating and drinking until he could distinguish between the black and the white by looking at them. Then Allah the Exalted revealed the words “from the dawn,” and then people understood that by threads was meant the night and the day.