664. Narrated ‘Abdullaah bin Mas’ood (radhi Allaahu anhu) : The Prophet (Sallallaahu Alaihi Wasallam) said,

The Adhaan pronounced by Bilaal should not stop you from taking Sahoor, for the pronounces the Adhaan at night, so that the one offering the late night prayer (Tahajjud) from among you might hurry up, and the sleeping from amongst you might wake up. It does not mean that Al-Fajr (dawn) or As-Subhu (morning) has started.” Then he (the Prophet (Sallallaahu Alaihi Wasallam)) pointed with his fingers and rised them up (towards the sky) and then lowered them (towards the earth) like this (Ibn Mas’ood imitates the gesture of the Prophet (Sallallaahu Alaihi Wasallam)).

[Saheeh Al-Bukhaari, Hadeeth No. 595, Vol. 1]

Source : The Book Of As-Siyaam (the fasting) – Chapter 8 – The timing of Saum (fast) begins with the dawn, and one is allowed to eat and drink till dawn, and the explanation of the dawn that marks the legal beginning of a Saum and about the beginning of the time of the Fajr (morning) prayer etc. Al-Lulu Wal-Marjan, Compiled by : Fuwad Abdul-Baqi, Translated by : Dr. Muhammad Muhsin Khan