Narrated by Ahmad ibn Muhammad ibn ‘Isa from al-Husayn ibn Sa’id from al-Husayn ibn Abi al-Husayn, who said: I wrote to Abu Ja’far عليه السلام; “May I be your ransom, your followers have differed regarding the time of the Fajr prayer. Some of them pray when the first, vertical dawn appears in the sky, and some of them pray when it spreads horizontally at the lower part of the horizon and becomes clear. I do not know which of the two times is better, so I may pray at that time. If you would, O my master, may Allah سبحانه وتعالى make me your ransom, teach me which of the two times is better, and set for me a standard regarding what I should do when the moon is present and the Fajr is not clearly visible until it reddens and brightens. And how should I act concerning the moon? And what is the limit of that in both travel and residence? May you do so, if Allah سبحانه وتعالى wills.” So He عليه السلام wrote back in his own handwriting (peace be upon him): "The Fajr, may Allah سبحانه وتعالى have mercy on you, is the white thread, and it is not the whiteness that rises upward. Do not pray, in travel or in residence, until it becomes clearly distinguishable. May Allah سبحانه وتعالى have mercy on you. Indeed, Allah سبحانه وتعالى has not placed His سبحانه وتعالى creation in confusion regarding this matter. He سبحانه وتعالى, the Exalted, said: ‘'Eat and drink until the white thread becomes clear to you from the black thread of the dawn'‘ (Surah Baqarah 2:187), the white thread is the Fajr which makes eating and drinking prohibited in fasting, and likewise it is the one that makes prayer obligatory."