«أَوْ لَا يُسَمَّى الرَّاوِي اخْتِصَارًا مِنَ الرَّاوِي عَنْهُ»
Or a narrator’s name is omitted for brevity by the one transmitting from him.
A chain may use an indeterminate phrase such as “so-and-so,” “a man,” “my teacher,” or “the son of someone” rather than naming the individual. This is al-mubham, a narrator left unidentified. It differs from knowing a specific name but lacking evaluative information.
An unnamed narrator is not proven unreliable simply by being unnamed. The problem is that his actual identity is not established, so the ordinary tools for checking integrity and precision cannot be applied with sufficient confidence.