إن لم يكن مشهورا عند البلغاء
If it is not famous among those skilled in rhetoric.
The qualification implies that a very famous line can announce its borrowing through its own recognizability. The same explicit warning that is needed for obscure poetry need not accompany every widely known citation.
This is a convention of literary recognition, not a guarantee that every audience knows the verse. When teaching an example, identifying its source remains useful even if the original readership would have recognized it at once.