بُلُوغُ الْمَرَامِ
Bulugh al-Maram 170
The Book of Prayer
وَلِلْحَاكِمِ فِي حَدِيثِ جَابِرٍ - رضى الله عنه - نَحْوُهُ, وَزَادَ فِي اَلَّذِي يُحَرِّمُ اَلطَّعَامَ: { إِنَّهُ يَذْهَبُ مُسْتَطِيلاً فِي اَلْأُفُقِ } وَفِي اَلْآخَرِ: { إِنَّهُ كَذَنَبِ اَلسِّرْحَان } [1] .
al-Hakim reported something similar to the above Hadith from Jabir and added to the type of dawn in which eating is forbidden: "it is spread widely in the horizon," and to the other type: "it has the shape of wolf's tail."
Sunnah.com reference: Book 2, Hadith 22