Kitāb al-Ghayba > Concealed Birth of the Imam of the Time (a.s) was not an extraordinary event
Hadith #16

ومثل ما حكيناه أيضا قصة أصحاب الكهف وقد نطق بها القرآن وتضمن شرح حالهم واستتارهم عن قومهم فرارا بدينهم. ولو لا ما نطق القرآن به لكان مخالفونا يجحدونه دفعا لغيبة صاحب الزمان عليه السلام وإلحاقهم به،لكن أخبر الله تعالى أنهم بقوا ثلاثمائة سنة مثل ذلك مستترين خائفين ثم أحياهم الله تعالى فعادوا إلى قومهم وقصتهم مشهورة في ذلك.

Likewise is the story of the “people of the cave” narrated by the Quran how their story unfolded and how they hid from their people and fled to save their religion. If the Quran had not spoken about it, our opponents would have rejected this in order to facilitate their denial of the occultation of the Master of the Age. However, Allah informed us that they remained three hundred years like that in hiding, in fear, and then Allah brought them back to life and they returned to their people. Their story is well known.