Kitāb al-Ghayba > Refutation of all sects, which believed in Imamate of someone other than Master of the Age (a.s)
Hadith #4

وأما من قال إن الأمر مشتبه فلا يدرى هل للحسن عليه السلام ولد أم لا، وهو مستمسك بالأول حتى يتحقق ولادة ابنه، فقوله أيضا يبطل بما قلناه من أن الزمان لا يخلو من إمام، لأن موت الحسن عليه السلام قد علمناه كما علمنا موت غيره. وسنبين ولادة ولده فيبطل قولهم أيضا.

As for the saying that the situation is confusing and it cannot be decided whether Imam Hasan Askari (a.s) sired a son or not, and until the birth of his son is established, we will adhere to the Imamate of Hasan; it is also invalidated through our assertion that no time can be without an Imam, because we know Hasan (a.s) is dead just as we know many other people to be dead, and we will prove the birth of his son as well, so their claim is also falsified.