Kitāb al-Ghayba > Concealed Birth of the Imam of the Time (a.s) was not an extraordinary event
Hadith #6

فإن قيل: كيف يجوز أن يكون للحسن بن علي عليه السلام ولد مع إسناده وصيته في مرضه الذي توفي فيه إلى والدته المسماة بحديث المكناة بأم الحسن بوقوفه وصدقاته وأسند النظر إليها في ذلك ولو كان له ولد لذكره في الوصية؟

If it is said: How Hasan Ibn Ali had a son while during his terminal illness, he addressed his final will regarding his trusts and monies to his mother, named Haditha and with patronymic of Umme Hasan, and entrusted his affairs to her? If he had a son, he would have mentioned him in his final will.