It is a ḥadīth in which a hidden defect (ʿilla) damaging to its soundness has been detected, even though it outwardly appears free of any such defect.
Recognising the ʿilla is helped by noting the narrator's solitary transmission, or his being contradicted by someone else, together with circumstantial indications that alert one familiar with this art to: an irsāl in what is apparently mawṣūl; a waqf in what is apparently marfūʿ; the intrusion of one ḥadīth into another; or some other lapse on the narrator's part, to the point where the suspicion of such a defect predominates.
He then gives his ruling accordingly; or he wavers and suspends judgement on the ḥadīth, since each of these factors is a barrier to declaring sound any ḥadīth in which it is found.1
Muqaddimat Ibn al-Ṣalāḥ, p. 71. An example: that which Ibn Jurayj narrated from Mūsā ibn ʿUqba, from Suhayl ibn Abī Ṣāliḥ, from his father, from Abū Hurayra, as a marfūʿ report, on the expiation of the sitting (kaffārat al-majlis). Imām al-Bukhārī said: this is a pleasing ḥadīth, except that it is defective. Wuhayb narrated it from Suhayl, from ʿAwn ibn ʿAbd Allāh, as his own words, and he said: this is the sounder, since no hearing by Mūsā ibn ʿUqba from Suhayl is recorded. End of quotation. Maʿrifat ʿUlūm al-Ḥadīth, p. 114. The difference between al-shādhdh and al-muʿallal: al-shādhdh is far subtler than al-muʿallal, since in al-muʿallal the defect can be identified, whereas in al-shādhdh it cannot be; the critic senses that the ḥadīth is in error but cannot produce a definitive argument for it. Because of its difficulty, no one has devoted an independent work to it. See: Maʿrifat ʿUlūm al-Ḥadīth, pp. 119 to 122; al-Nukat al-Wafiyya, 1/455; Fatḥ al-Mughīth, 2/10; Sharḥ al-Taqrīb by al-Sakhāwī, p. 138; Tadrīb al-Rāwī, 1/233. This is according to the view of ʿAllāmah al-Ḥākim and those who followed him. On the view of the other muḥaddithūn, it is not a condition of al-shādhdh that its defect cannot be identified, so that it may combine with al-muʿallal and with others, as appears from the examples given for al-shādhdh, and as is understood from the definitions of al-Khalīlī, Ibn al-Ṣalāḥ and others. And Allāh knows best. ↩
When a narrator's solitary transmission is suspected, others' parallel narrations are mutābaʿa or shāhid; the pursuit of these is al-iʿtibār.
Al-shādhdh is what a single narrator transmits alone, raising doubt in the mind of the critic; this is also al-munkar, and the muḥaddithūn treat the two as one.
Al-mudraj is the mixing of what belongs to one speaker with what belongs to another without indication. It falls into mudraj al-matn (more common) and mudraj al-sanad.
A ḥadīth in which a narrator has been replaced with another, whether one narrator, several, or even the entire chain, is al-maqlūb (literally, that which has been inverted).
