Musannaf Ibn Abi Shayba 3:988

Yahya ibn Abi Bukayr narrated to us, he said: Fudayl ibn Marzuq informed us, he said: I asked Jabir al-Ju'fi about this verse: "Until the white thread becomes distinct to you from the black thread of the dawn." He said: Sa'id ibn Jubayr said: It is the redness of the horizon.