Not Every Ḍaʿīf Ḥadīth Qualifies as a Corroborator

Imām Ibn as-Salāh (rahimahullāh) said in his ʿUlūm al-Hadīth book, "Not every weakness in a hadith is eliminated by the hadith coming from several lines of transmission. Rather, the situation varies...

One weakness (ḍuʿf) which the coming of a hadith via several lines of transmission eliminates is the weakness which arises from some deficiency in the retention (hifz) of its transmitter, when he is otherwise veracious and pious (ahl as-sidq wa ad-diyānah). When we see that the hadith he related also comes from another line of transmission, we realize that it is one of the hadith that he had retained properly and his accuracy in it was not impaired.

Likewise, when the weakness of the hadith is on account of irsāl (what a tābiʿī narrates directly from the Prophet ﷺ), it disappears because of something along these lines - as in the case of the Mursal hadith which an expert authority transmits as Mursal - since it contains just a small weakness which disappears through its relation from another line of transmission.

There are also types of weakness which are not eliminated by something similar to that, on account of the severity of the weakness and the failure of this "bone-setter" to reduce the fracture in it and mend it. An example of this is the weakness which arises from the transmitter being accused of falsehood (muttaham bi al-kazib) or from the hadith being anomalous (shādh). This is an outline, the particulars of which are acquired through practice and study." (33-34, Dār al-Fikr)