Musannaf Ibn Abi Shayba 3:2470

Abu Bakr narrated to us, he said: Mu'tamir narrated to us, from Layth, from Tawus, that he said regarding a man who prays two rak'ahs of the obligatory prayer alone in the mosque, then the prayer is established: He said: He should pray with them, and he should not count it [the first prayer] as sufficient.