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 the one who has already prayed it to lead those who have not, because Muath, may Allaah be pleased with him, used to pray with the Prophet (sallallaahu alaihi wa sallam) then he would return to the people and perform the same prayer with them.  And Abu Dawud narrated in his Sunan that the Prophet (sallallaahu alaihi wa sallam) led a number of Companions, may Allaah be pleased with them, in the prayer of fear (Salatul-Khawf) consisting of two Rakahs, then he prayed two Rakahs with another group, then he made the Taslim.  This is one of the two opinions expressed by Imam Ahmad, and the opinion of Ata, Al-Awzai, Ash-Shafii and Ibn Al-Munthir. And Allaah knows best.
 Al-Bukhari no. 700  Abu Dawud no. 1244. [The claim Allamah Tahawi has made as regards this Hadith that it is Mansukh, is not acceptable because this claim has no supporting proofs with it. Allamah Sindhi said that this Hadith strongly proves that obligatory prayer can be offered behind an Imam offering his voluntary prayer, and the persons opposing his view have no valid reasons for that. (Aunul-Mabud, Vol. 4, p. 90)]