Yoma 6:7-8

Yoma 6:7

The Kohein Gadol came to the bull and the goat that were waiting to be burned. He tore them open and removed the sacrificial fats. He put these fats on a tray and burned them on the altar. He wrapped the carcasses on poles so that they could be carried out to the place where they would be burned. The clothes of those who removed them became ritually unclean as soon as they left the confines of the Temple courtyard; Rabbi Shimon said the clothes became unclean when the major portion of the carcasses caught fire.

Yoma 6:8

They informed the Kohein Gadol when the scapegoat reached the wilderness, which they knew because they had set up relay stations. They waved scarves to indicate that the scapegoat had reached the wilderness. Rabbi Yehuda questioned this because they had a sign: from Jerusalem to the edge of the wilderness was three mil (a little over two miles), so they could walk a mil and back, then wait the time it took to walk a mil, and they would know that the goat had reached the wilderness. Rabbi Yishmael said there was another sign: there was a red thread tied to the door of the Sanctuary that would turn white when the goat reached the wilderness, as per Isaiah 1:18, “Though your sins are like scarlet, they will be as white as snow.”