Narrated to me my father...
Narrated to me my father; he said: narrated to me Saad bin Abdullah from Muhammad bin Isa bin Ubaid from Ali bin Hakam and Ali bin Hasan from Nafe Warraq from Harun bin Kharja from Harun bin Saad Ajali that he said: “Ismail, about who you thought that he would be the Imam after Imam Ja’far Sadiq (a.s.), died. And Imam Ja’far Sadiq (a.s.) has also reached old age and he will also pass away one of these days. After that you will become without any Imam. I could not give him any reply.
So I mentioned his statement to Imam Ja’far Sadiq (a.s.). He said: “I am surprised at this man. By Allah, if Imamate stops, the days and the nights will come to an end. When you meet him next time tell him that Musa Ibne Ja’far will be the Imam after me. After him, his son will succeed him, if Allah wills.” In this narration Imam Ja’far Sadiq (a.s.) has stated under oath that the matter of Imamate cannot end till the days and nights continue to follow each other.
Hence, the period of Fatara is allowed between messengers because among the duties of the messengers is that they bring a new Shariat, they renew the past Shariat and through some commands they abrogate some laws. But the period of Fatara is not allowed between prophets and Imams because they neither bring a new Shariat nor is it their responsibility to abrogate or renew.
And we know that during the periods between Prophet Nuh and Prophet Ibrahim (a.s.), between Prophet Ibrahim and Prophet Musa (a.s.), between Prophet Musa and Prophet Isa (a.s.) and between Prophet Isa and (S) a large number of prophets and successors were sent and their duty was to remind the people about the divine commands and to protect the bequests, books and sciences that have reached them through the messengers.
And all the messengers from them had propagated the matters that reached them from Allah, the Mighty and Sublime and the successors of those prophets guarded those sciences and wills. When the Almighty Allah ended the prophethood with (S) it was necessary that there should be a guide and a successor who would protect the religion of truth and propagate the faith because the earth cannot remain without Divine Proof.