Kitāb al-Ghayba > Concealed Birth of the Imam of the Time (a.s) was not an extraordinary event
Hadith #18

وقد كان من أمر صاحب الحمار الذي نزل بقصته القرآن وأهل الكتاب يزعمون أنه كان نبيا فأماته الله تعالى مائة عام ثم بعثه وبقي طعامه وشرابه لم يتغير.وكان ذلك خارقا للعادة. وأمثال ما قلناه كثيرة مما رواه أصحاب السير والتواريخ من ملوك الفرس وغيبتهم عن أصحابهم مدة لا يعرفون خبرهم ثم عودهم وظهورهم لضرب من التدبير،وإن لم ينطق به القرآن فهو مذكور في التواريخ. وكذلك جماعة من حكماء الروم والهند قد كانت لهم غيبات وأحوال خارجة عن العادات لا نذكرها لأن المخالف ربما جحدها على عادتهم جحد الأخبار وهو مذكور في التواريخ.

If all these events are well known, how can they reject the occultation of the Master of the Age? Except that the opponent may be an atheist, nihilist, negating all these as impossibilities, in which case we will not discuss with him the subject of occultation, but rather our discourse with him will change to the very existence of God and that this is within the bounds of God’s infinite power. Our discourse with regard to occultation is with someone who is a Muslim and confesses that this is within the realm of power of God and we are showing similar examples to him. Similar examples of this, narrated by historians and biographers are many such as in the stories of Persian kings and their hiding from their people for a period in which they do not know their whereabouts and they return to show their purpose from the enterprise. Though the Quran has not spoken of this, it is chronicled in history. Likewise, a number of rulers of Rome and India had hidings and extraordinary events, which we will not mention, because the opponents may reject them as is their habit in order to reject the traditions.