If the disagreement consists in the addition of a narrator within the chain, and the one who did not include the addition is more proficient than the one who did, this is al-mazīd fī muttaṣil al-asānīd (the surplus narrator within an otherwise connected chain).
Its condition is that there be an explicit statement of audition at the place of the addition; otherwise, if the chain there is muʿanʿan (say), the addition is given preference.1
Sharḥ al-Nukhba, p. 64. An example: ʿAbd Allāh ibn al-Mubārak narrated, from Ibn Jābir, from Busr ibn ʿUbayd Allāh, from Abū Idrīs, from Wāthila, from Abū Marthad, from the Prophet ﷺ: "Do not pray towards the graves, and do not sit upon them." Abū Ḥātim said: it is held that Ibn al-Mubārak erred in this ḥadīth, inserting Abū Idrīs al-Khawlānī between Busr and Wāthila. ʿĪsā ibn Yūnus, Ṣadaqa ibn Khālid and al-Walīd ibn Muslim narrated it from Ibn Jābir, from Busr, who said: "I heard Wāthila narrate from Abū Marthad al-Ghanawī, from the Prophet ﷺ." Abū Ḥātim said: Busr frequently narrates from Abū Idrīs, and Ibn al-Mubārak made a slip, supposing that this ḥadīth was among those Busr narrated from Abū Idrīs from Wāthila. ↩
An omission of two or more consecutive narrators at any point in the chain — this is al-muʿḍal.
Al-muttaṣil (also called al-mawṣūl) is that whose chain of transmission is unbroken: every narrator has heard the report from the one above him, all the way back to its source.
When each of two peers narrates from the other, this is al-mudabbaj (literally, embellished or paired); al-ʿIrāqī, following al-Dāraquṭnī, did not require the two narrators to be peers.
Among the muḥaddithūn, al-majhūl is a narrator whose condition is not known and who is not known for his pursuit of knowledge; he splits into majhūl al-ḥāl (al-mastūr) and majhūl al-ʿayn.
