Kitāb al-Ghayba > Refutation of all sects, which believed in Imamate of someone other than Master of the Age (a.s)
Hadith #5

وأما من قال إنه لا إمام بعد الحسن عليه السلام، فقوله باطل بما دللنا عليه من أن الزمان لا يخلو من حجة لله عقلا وشرعا. وأما من قال إن أبا محمد عليه السلام مات ويحيا بعد موته، فقوله باطل بمثل ما قلناه،لأنه يؤدي إلى خلو الخلق من إمام من وقت وفاته عليه السلام إلى حين يحييه الله تعالى.

As for the belief that there is no Imam after Hasan (a.s): It is invalid due to our rational as well as Shari proofs that no time can be devoid of an Imam. The belief that Abu Muhammad passed away and he will come back to life after his death, is also invalid, because it suggests lack of an Imam from the time of his death until Allah brings him back to life.