the narration from that reliable authority. Suppose, for instance, that an untrustworthy person narrates from al-Zuhrī—even though al-Zuhrī himself is reliable. In such a case we must look for someone who has followed this narrator in transmitting from al-Zuhrī. If we locate such a corroborator, the report is strengthened. Alternatively, we might find another ḥadith through a different chain altogether that supports the same content; this is called a shāhid (supporting report). Our search through the various lines of transmission with the aim of locating a mutābiʿ (corroborator) or a shāhid is known as iʿtibār (comparative analysis). This, then, is the meaning of Imām Aḥmad’s words, “la-aʿtabiruhu”—namely, that I examine it to see whether it has a corroborator or a supporting report.

Hence Imām Aḥmad cites ʿAbd-Allāh ibn Lahīʿah very frequently in the Musnad. Those who heard from Ibn Lahīʿah before the fire that destroyed his books considered him a sound authority, whereas those who heard from him afterward deemed his transmissions doubtful and unreliable, for his precision deteriorated once his books were lost. If we cannot tell whether a particular student heard from him before or after the fire, we suspend judgment rather than pronounce decisively; but in the second category—where it is certain the hearing took place after the fire—we rule the narration erroneous.

Just as the scholars use the ḥadith of someone with a weak memory for corroboration and iʿtibār, they also declare certain reports of an upright, accurate transmitter to be weak whenever evidence shows he erred in that specific narration. They call this pursuit “the science of hidden defects (ʿilal al-ḥadith),” one of their noblest disciplines. A ḥadith may be related by a meticulous, trustworthy narrator, and yet the experts discern his mistake—either for an obvious reason, as when they established that the Prophet, peace and blessings be upon him, married Maymūnah while he was in a state of ritual release, not iḥrām (1), and that he prayed two units inside the Kaʿbah (2); therefore they judged the report of Ibn ʿAbbās that he married her while in iḥrām (3) and that he did not pray inside the House to be erroneous. Likewise, the claim that he performed four—