بسم الله الرحمن الرحيم
al-Tābiʻī al-Kabīr - Sulaym (ra) has mentioned in his book, what Prophet (saww) has mentioned in his "Will". Nothing has been mentioned in it about ‘al-Mahdiyoon’ or ‘Ahmad al-Hassan’. Let's see, what Prophet (saww) has mentioned in his "Will".
وسمى من يكون من أئمة الهدى الذين أمر الله بطاعتهم إلى يوم القيامة. فسماني أولهم ثم ابني هذا - وأدنى بيده إلى الحسن - ثم الحسين ثم تسعة من ولد ابني هذا - يعني الحسين
Imam Ali (as) said: [...] and he [i.e. Prophet (saww) while dictating the "Will"] told the names of the Imams (as) who are guides, whose obedience Allah (swt) has commanded till the Day of Judgement. In these first is my name, and the second is this my son [pointing towards Hassan (as)]. After him, Hussain (a.s) and then nine from my this son Hussain (as).
Source: Kitāb Sulaym ibn Qays al-Hilālī by al-Tābiʻī al-Kabīr, page 211,
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