«لِكَذِبِ وَاحِدٍ مِنْهُمَا لَا بِعَيْنِهِ»
“Because one of the two statements is false, without specifying which one.”
When the student affirms direct hearing and the teacher categorically says he never narrated it, those two claims cannot both stand in their stated form. This creates a decisive contradiction for that particular route of transmission.
It does not automatically identify either the pupil or teacher as the liar. Establishing personal blame requires separate evidence. The immediate issue is the acceptability of this contested attribution. An independently established chain for the same content must be considered on its own merits.