Muhammad bin Abd al-Rahman bin Thawban (may Allah have mercy on him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "There are two dawns. As for the one that is like the tail of a wolf, it neither makes anything forbidden nor makes anything lawful. But the elongated one that spreads across the horizon makes the prayer lawful and makes food forbidden."
This is mursal (the Successor narrates directly from the Prophet), and it has also been narrated with a connected chain mentioning Jabir bin Abdullah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8002