Khabar al-Āḥād and Ijmāʿ

Imam al-Manṣūr Billāh ʿAbdullāh ibn Ḥamzah (d. 614 AH) discusses in Ṣafwat al-Ikhtiyār fī Uṣūl al-Fiqh whether a solitary report (khabar al-āḥād) can overturn or undermine an established consensus (ijmāʿ).

He relates that his teacher, Shaykh Abū ʿAlī al-Ḥasan ibn Muḥammad al-Raṣṣāṣ (d. 584 AH), held that if ijmāʿ becomes established among the scholars of an age and no disagreement is known, then a later solitary hadith narrating a contrary view does not invalidate that consensus. Shaykh Abū ʿAlī argued that ijmāʿ is known with certainty, whereas khabar al-wāḥid only yields probabilistic knowledge (maḍnūn), so certainty cannot be abandoned for what is merely probable.

As an example, Imam ʿAbdullāh ibn Ḥamzah mentions the claimed ijmāʿ that anything nourishing entering the throat breaks the fast, despite a narration attributed to Abū Ṭalḥah stating that swallowing hail (al-barad) does not break the fast. He then notes that Shaykh Abū ʿAlī adopted this approach and argued for it, using the example as support for the principle that a solitary report does not overturn an established consensus.

Imam ʿAbdullāh ibn Ḥamzah, however, disagreed with his teacher on this point. He maintained that khabar al-āḥād can undermine a claimed ijmāʿ, arguing that even the apparent meaning of the Qurʾān (dhāhir al-kitāb) may at times be specified or departed from on the basis of solitary reports. Therefore, he held that a claimed consensus is not necessarily stronger than a valid khabar al-āḥād.

Source

Ṣafwat al-Ikhtiyār fī Uṣūl al-Fiqh, pp. 248–249, by Imam al-Manṣūr Billāh ʿAbdullāh ibn Ḥamzah (d. 614 AH).