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, for example, muʿanʿan (that is, narrated using ʿan, 'on the authority of', rather than an explicit hearing-form such as ḥaddathanā or samiʿtu), 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. Busr had in fact heard this ḥadīth from Wāthila himself, since the people of Shām are better acquainted with their own ḥadīth. End of quotation. ʿIlal Ibn Abī Ḥātim, no. 213. ↩
An omission of two or more consecutive narrators at any point in the chain is al-muʿḍal, one of the four kinds of rejected report rooted in a break in the chain.
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.
An omission from the start of the isnād by the compiler's action is al-muʿallaq, one of the four kinds of rejected report rooted in a break in the chain.
Al-mawḍūʿ is that which has been concocted and fabricated. It is known by four indications, and may not be narrated to one who knows its condition without disclosure.
