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
It is narrated from Muhammad bin Abd al-Rahman bin Thawban (may Allah have mercy on him) that the Messenger of Allah (peace and blessings be upon him) said: "Fajr (dawn) is of two kinds," and the rest of the hadith is the same. (It has also been narrated through another chain as marfu' and as mawquf.)
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1766
It is narrated from Sayyiduna Ibn Abbas (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: a fajr in which eating is permissible and prayer is forbidden, and a fajr in which prayer is permissible and eating is forbidden."
Abu Ahmad narrated it thus as musnad (attributed to the Prophet), while others narrated it as mawquf, and the mawquf is more correct.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1767
It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both): "Fajr (dawn) is of two kinds: the fajr that rises while it is still night, in which eating and drinking are permissible and prayer is not permissible; and the other fajr, in which prayer is permissible and eating and drinking are forbidden, and it is the one that spreads over the tops of the mountains."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1768