Among hadith scholars are masters of precision, such as Shu‘ba, Mālik, al-Thawrī, Yahyā ibn Sa‘īd al-Qattān, and ‘Abd al-Rahmān ibn Mahdī. They attained the highest degree of precision and retention, unlike those below their rank. These scholars may consider a narrator weak because of the many errors in his reports. Yet, if his reports are predominantly sound, they can serve for comparison and corroboration. Multiple chains of transmission strengthen one another and may even yield certain knowledge, even if the transmitters are wicked and sinful. How much more so when they are upright scholars whose reports nevertheless contain many errors.
An example is ‘Abd Allāh ibn Lahī‘a, who was among the leading Muslim scholars. He served as a judge in Egypt and narrated many hadiths. However, his books were burned, so he began narrating from memory, and many errors entered his reports, although most remained sound. Ahmad said, “I may record a man’s reports for comparison with other reports—as in the case of Ibn Lahī‘a.”
As for someone known to lie deliberately, some scholars would transmit nothing from him. This was the approach of Ahmad ibn Hanbal and others. In his Musnad, Ahmad did not narrate from anyone he knew to lie deliberately. He did, however, narrate from those he knew to make mistakes, for comparison and corroboration.
Other scholars would listen to the reports of narrators who lied, saying that they