Sayyiduna Abdullah bin Mas’ud (may Allah be pleased with him) narrates this statement of the Prophet (peace and blessings of Allah be upon him):“Bilal gives the adhan at night (i.e., a little before true dawn), so that the sleeping person may be alerted and the one performing nawafil may return (and partake in suhoor). True dawn does not appear like this.”The Prophet (peace and blessings of Allah be upon him) indicated with his two index fingers.“Rather, true dawn appears like this.”The Prophet (peace and blessings of Allah be upon him) indicated with his palm. (Imam Ibn Hibban, may Allah have mercy on him, says:) The narration of Sayyiduna Abdullah bin Mas’ud (may Allah be pleased with him) from the Prophet (peace and blessings of Allah be upon him):“Bilal gives the adhan at night (i.e., a little before true dawn) so that he may alert the sleeping person and the one performing nawafil may return (to partake in suhoor).”In this narration, there is a clear statement that Sayyiduna Bilal (may Allah be pleased with him) used to give the adhan before true dawn so that he could wake up the sleeping person and the one performing tahajjud (after completing nawafil) could return to partake in suhoor. He did not give this adhan for the Fajr prayer. So, when there were two mu’adhdhins in the mosque and one of them would give the adhan a little before true dawn, as we have mentioned, then the other would give the adhan for the Fajr prayer at the time of true dawn, then this is permissible. However, if someone gives the adhan for the morning prayer before true dawn, then it is obligatory upon him to give the adhan again for the morning prayer, because it is not authentically established that both mu’adhdhins ever gave the adhan before true dawn.