Ibn Majah

1؎: Those who remained in the mosque until the prayer was completed obeyed the command of the Prophet (sallallahu alayhi wa sallam). The statement of the Prophet (sallallahu alayhi wa sallam) is that when the call to prayer (adhan) is given in the mosque, then one should not leave without performing the prayer. If a person is the imam of another place and there is a severe necessity, then it is permissible for him to leave; however, it is better that he performs the prayer before leaving. According to the hadith scholars (muhaddithin), it is permissible for a person to perform the prayer in congregation, and then in the same prayer, lead others as imam. Mu'adh (radi Allahu anhu) used to do this: his first congregational prayer would be obligatory (fard), and the second prayer in which he led others as imam would be voluntary (nafl), while the followers (muqtadis) behind him would be performing their obligatory prayer.