I say, with Allah's guidance: It starts from the appearance of the widespread dawn in the horizon - known as the true dawn - until sunrise. There are two dawns: a false dawn - called by the Arabs the tail of the wolf - which is the whiteness that appears vertically in the sky followed by darkness, and a true dawn: which is the whiteness spread across the horizon; as the Prophet (peace be upon him) said: 'Do not be deceived by Bilal's call to prayer or the vertical whiteness of the horizon until it spreads horizontally,' as narrated by Hammad using his hands to illustrate, meaning horizontally, in Sahih Muslim 2:770. In another narration: 'Bilal's call should not prevent any of you from your pre-dawn meal, for he calls to awaken the sleeper and to return the one praying, and it is not to say - meaning the morning - like this, or like this, but until he says: like this and like this, meaning vertically but like this meaning horizontally,' in Sahih Ibn Khuzaymah 3:210. See: Al-Tahtawi's commentary on Al-Durr Al-Mukhtar 1:173, and Allah knows best.