Quran:2:187:- ...and eat and drink, until the white thread of dawn appear to you distinct from its black thread;....

Narated By Talq ibn Ali al-Yamami : The Apostle of Allah (pbuh) said: Eat and drink; let not the white and ascending light prevent you from (eating and drinking); so eat and drink until the red light spreads horizontally.

narrated on the authority of Sulaiman Taimi with the same chain of transmitters and, at the end, it was said that the first Adhan was meant to awaken those who were in slumber amongst them and in order to make them turn who stand in (prayer) among them (towards food at the commencement of the fast). Jarir (one of the narrators) said that the Messenger (may peace be upon him) did not say like this but he said like it (true dawn) that the streaks of (true dawn) are horizontal and not vertical.

Samura b. Jandub reported Muhammad (may peace be upon him) as saying. The call of Bilal may not mislead any one of you (and he may, under the wrong impression gathered from it, refrain) from taking meal before the commencement of the fast (for the streaks) of this whiteness (which are vertical indicate the false dawn and the true dawn with which the fast commences is that when the streaks of light are) spread.

Samura b. Jundub (Allah be pleased with him) reported Allah's Messenger (may peace be upon him) as saying: The Adhan of Bilal may not mislead you with regard to your food at the commencement of the fast, nor the vertical (streaks) of whiteness in the horizon (for it is an indication of false dawn). You should stop eating (food) till (the whiteness) spreads like it. Hammad narrated it and with the gesture of his band he explained the horizontal position (of the streaks of light).

Narated By 'Abdullah bin Mas'ud : The Prophet said, "The Adhan pronounced by Bilal should not stop you from taking Suhur, for he pronounces the Adhan at night, so that the one offering the late night prayer (Tahajjud) from among you might hurry up and the sleeping from among you might wake up. It does not mean that dawn or morning has started." Then he (the Prophet) pointed with his fingers and raised them up (towards the sky) and then lowered them (towards the earth) like this (Ibn Mas'ud imitated the gesture of the Prophet). Az-Zuhri gestured with his two index fingers which he put on each other and then stretched them to the right and left. These gestures illustrate the way real dawn appears. It spreads left and right horizontally. The dawn that appears in the high sky and lowers down is not the real dawn.

Sahih Bukhari3:31:140:-
Narated By 'Adi bin Hatim : When the above verses were revealed: 'Until the white thread appears to you, distinct from the black thread,' I took two (hair) strings, one black and the other white, and kept them under my pillow and went on looking at them throughout the night but could not make anything out of it. So, the next morning I went to Allah's Apostle and told him the whole story. He explained to me, "That verse means the darkness of the night and the whiteness of the dawn."

Sahih Bukhari3:31:141:-
Narated By Sahl bin Saud : When the following verses were revealed: 'Eat and drink until the white thread appears to you, distinct from the black thread' and of dawn was not revealed, some people who intended to fast, tied black and white threads to their legs and went on eating till they differentiated between the two. Allah then revealed the words, 'of dawn', and it became clear that meant night and day.

Sahih Muslim:6:2408:-Samura b. Jundub reported Allah's Messenger (may peace be upon him) as saying: The Adhan of Bilal should not mislead you nor the whiteness (of the pillar) of dawn, for it is not the whiteness of the true dawn, but that of the false dawn which is vertical like a pillar and you can eat food till the streaks of whiteness spread like it(Horizontal).

Abu Dawood:2:534:-
Narated By Bilal : The Apostle of Allah (pbuh) said to Bilal: Do not call adhan until the dawn appears clearly to you in this way, stretching his hand in latitude.

Abu Dawood:7:2448:-
Narated By Hafsah, Ummul Mu'minin : The Apostle of Allah (pbuh) said: He who does not determine to fast before dawn does not fast.