Kitāb al-Ghayba > Concealed Birth of the Imam of the Time (a.s) was not an extraordinary event
Hadith #7

قيل: إنما فعل ذلك قصدا إلى تمام ما كان غرضه في إخفاء ولادته وستر حاله عن سلطان الوقت ولو ذكر ولده أو أسند وصيته إليه لناقض غرضه خاصة وهو احتاج إلى الإشهاد عليها وجوه الدولة وأسباب السلطان وشهود القضاة ليتحرس بذلك وقوفه ويتحفظ صدقاته ويتم به السترعلى ولده بإهمال ذكره وحراسة مهجته بترك التنبيه على وجوده. ومن ظن أن ذلك دليل على بطلان دعوى الإمامية في وجود ولد للحسن عليه السلام كان بعيدا من معرفة العادات. وقد فعل نظير ذلك الصـادق جعفـر بن محمد عليه السلام حين أسند وصيته إلى خمسة نفر أولهم المنصور إذ كان سلطان الوقت ولم يفرد ابنه موسى عليه السلام بها إبقاء عليه وأشهد معه الربيع وقاضي الوقت وجاريته أم ولده حميدة البربرية وختمهم بذكر ابنه موسى بن جعفر عليه السلام لستر أمره وحراسة نفسه ولم يذكر مع ولده موسى أحدا من أولاده الباقين لعلمه كان فيهم من يدعي مقامه من بعده ويتعلق بإدخاله في وصيته،ولو لم يكن موسى عليه السلام ظاهرا مشهورا في أولاده معروف المكان منه وصحة نسبه واشتهار فضله وعلمه وكان مستورا لما ذكره في وصيته ولاقتصر على ذكر غيره كما فعل الحسن بن علي والد صاحب الزمان عليه السلام.

We will say: He did that in order to advance the purpose he had in hiding his birth and concealing him from the rulers of the time. Had he mentioned his son and addressed his final will to him, he would have breached his sole purpose. He needed the government officials, yeomen of the king and witnesses of the judges to refer to her in order to safeguard his trusts and monies and also to maintain secrecy of existence of his son and to protect his life by not mentioning him. Anyone who thinks that this is the proof of the falsity of the Imamiyah belief, he is not familiar with practices in the world. Imam Sadiq Ja’far Ibn Muhammad did the same when he addressed his final will to five people, the first of whom was the ruler of that time, Mansur. He did not exclusively address his son Musa (a.s) in order to protect him. He addressed Rabi, the judge of the time, and his concubine Hamida Barbariyya, and he mentioned his son Musa Ibn Ja’far (a.s) last in order to hide his position and protect his life. He did not mention along with his son Musa (a.s) any other of his sons. Perhaps it was to exclude the possibility of someone claiming the position of Imamate after him on the basis of his inclusion among addressees of the final will. And had not been Musa (a.s) prominent and well known amongst his sons, and his position and relationship to him famous, and his scholarship and erudition well established, and had he been unknown, Imam Sadiq (a.s) would not have mentioned him in his final will and would have sufficed on the others, as did Hasan Ibn Ali, the father of the Master of the Age.