It is narrated from Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Fajr (dawn) is of two kinds. The fajr that is like the tail of a wolf: in it the prayer is not valid, nor does eating become forbidden. And the fajr that spreads straight along the horizon: it makes the prayer permissible and makes eating forbidden."
It has been narrated thus with this chain as mawsul (connected), and it has also been narrated as mursal, which is more correct.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1765