The term mudallas refers to concealment, and is often used in trade where defects in goods being sold are concealed.
A hadith that is mudallas is one which is weak (da'eef) due to the uncertainty caused by the concealment occurring in the narration process.
Typically this occurs where a reporter has concealed the identity of a narrator in some manner.
The famous scholar Ibn al-Salah describes two types of tadlis:
- Tadlis al-Isnad: A person narrates something he did not hear from a narrator whom he met, or from someone he did not meet whilst creating the impression that he heard the hadith in person. Phrases such as "on the authority of" or "he said" often indicate this form of concealment.
- Tadlis al-Shuyukh: The reporter mentions the name of the narrator however uses a less well-known name or nickname so as not to disclose the narrator's identity.