There were two calls for salaat ul-fajr (prayer) at the time of Allaah's Messenger (peace be upon him), the first just before dawn—for those who are awake performing night prayers( Qe iaam Al-lyl) so as for them to either rest for Fajr or have food if they intend to fast the next day, and for those who are asleep in order they may begin to arouse and be ready for Fajr( since it is preferred as early as possible)—and the second at the actual time of fajr to indicate the appropriate time and to call people to the prayer, among other benefits of the adhaan.
Regarding the second point, jumhoor al-'ulemaa' (majority of the scholars) agree that the statement (within adhaan for fajr) "prayer is superior to sleeping" should be said in the second call to prayer. Hence, the caller or mu'athin should adhere to the saying phrase and recite it even if people are inside the mosque. It contains benefits which might be slip one's mind. Moreover, it reminds people of the very fact: "prayer is superior to sleep" which concerns the immediate day as well as all forthcoming days. Muslims benefit from it even when they are already in the mosque and gain the reward of repeating it after mu'athin, as per the saying of the Prophet :
"If you hear al-Mu'athin, say [i.e, repeat] as he says"
which is hadith sahih. Perhaps women and children who live near the masjid may benefit as well, and even the Muslims among the jinn. Additionally, there may be some people who feel sleepy inside the mosque or those who have nodded off while sitting. Thus, Muslims should preserve and practice this sunnah and not abandon it. May Allaah make us all successful in following the sunnah and vigilant in adhering and preserving it.
Sheikh Muhammed Salih Al-Munajjid