The Book of Occultation — al-Ṭūsī · Hadith 331 (#Ḥadīth #1)

وأما الذي يدل على أنه يكون من ولد الحسين عليه السلام فالأخبار التي أوردناها في أن الأئمة اثنا عشر وذكر تفاصيلهم هي متضمنة لذلك، ولأن كل من اعتبر العدد الذي ذكرناه قال: المهدي من ولد الحسين عليه السلام. وهو من أشرنا إليه. ويزيد ذلك وضوحا:

As for the reports, which say that Imam Mahdi is from the descendants of Imam Husain are traditional reports based on the fact that Imams are twelve and their explanation includes this meaning as the number [of twelve Imams] we mentioned is authentic and in fact it is believed that Mahdi is from the descendants of Imam Husain and this is what we have hinted to make clearer the meaning and more than that which is mentioned before, is the following report:

Gradings are classical jarḥ-wa-taʿdīl, name-matched to a rijāl index (not individually re-verified).