The designation by the caliph al-Ma'mūn of the ‘Alid imām ‘Alī al-Riọā as his successor was an extraordinary event by the standards of the time. Based on a letter that al-Ma'mūn is said to have written to the ‘Abbāsids after the death of al-Riḍā, in which he appears to confess that his decision to designate the latter had been influenced by apocalyptic beliefs about the end of the ‘Abbāsid dynasty, Madelung has argued that the caliph's decision can indeed be explained in terms of those beliefs. This article seeks to demonstrate that the relevant passage of the confession is unlikely to have been part of the original document. It goes on to suggest that the passage was probably added by Ibn Ṭāwūs (d. 664/1266), the document's redactor, in an attempt to support his own perceived role as the pre-Mahdic ‘Alid destined to rule after the fall of the ‘Abbāsids.