If a Person leads a People in an Obligatory Prayer which He has already performed
Question :
Is it permissible for one who has already performed the obligatory prayer to lead another congregation in the same prayer?
Answer:
The scholars have differed regarding whether this is permissible or not, but it appears to us that it is permissible for a person who is praying his obligatory prayer to lead those who have already prayed it, and for one who has already prayed it to lead those who have not, because Mu'ath, may Allah be pleased with him, used to pray with the Prophet then he would return to his people and perform the same prayer with them.
And Abu Dawud narrated in his Sunan that the Prophet led a number of his Companions, may Allah be pleased with them, in the prayer of fear (Salatul-Khawf) consisting of two Rak'ahs, then he prayed two Rak'ahs with another group, then he made the Taslim.
Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 2 Page 175