Shaykh Saduq has narrated in Kamaaluddin:2 It is narrated from Ameerul Momineen (a.s.) that he said: A man from my descendants would appear in the last period of time. Till he said: He would have two names, one confidential and the other, popular. His secret name is Ahmad… and so on. In Ghaibat3 of Shaykh Toosi, it is narrated from Huzaifah that he said: I heard the Messenger of Allah (s.a.w.a.) mention the Mahdi. He said: They will pay allegiance to him at Rukn and Maqam. His name is Ahmad, Abdullah and Mahdi. Thus, these names are for him.
It is mentioned in Taareekh Ibne Khashshaab4 and other books that the Imam is having two names: apparently it denotes the two names of the Messenger of Allah (s.a.w.a.).