Silsilat al-Ahadith al-Sahihah

Sayyiduna Jabir bin Abdullah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "There are two types of dawn: one is the false dawn, in which the light rises upwards like the tail of a wolf, not spreading horizontally; and the other is the (true) dawn, in which the light spreads horizontally, not vertically."