As for weak narrations, they recognize two categories:
A weak report whose deficiency does not prevent acting upon it; this corresponds to the ḥasan grade in al-Tirmidhī’s terminology. And a weak report whose deficiency demands that it be abandoned; this is the extremely weak (al-wāhī) report.

A narrator may be judged weak because he frequently errs, even though the greater part of what he transmits is sound. In that case they will still transmit his hadiths for comparison and corroboration (1). When the routes of transmission become numerous, they reinforce one another to the point that certainty may be attained through them—even if the transmitters were flagrant sinners and wrongdoers. So how much more so when the narrators are upright scholars who merely happen to make many mistakes. One such example (2) is ʿAbdullāh ibn Lahīʿah: he was among the leading scholars of the Muslims and served as the judge of Egypt, possessing a vast trove of hadiths. When his books were lost in a fire he began narrating from memory, and many errors crept into his reports, though most of his hadiths remain sound. Aḥmad said, “I still write down a man’s hadith for corroborative purposes, as I do with Ibn Lahīʿah.”

In another passage Shaykh al-Islam Ibn Taymiyyah—may Allah have mercy on him—explained why a weak hadith may be strengthened through multiple chains, the condition for that, and the obligation to adhere to this principle. He said (13/347):