What is the ruling on a mufti or imam saying: “It is not reported thus from the Messenger of Allah”?

And if the mufti or imam says: “I do not know that this was reported from the Messenger of Allah (peace and blessings be upon him).” Does he mean within the limit of his knowledge?
If he says: “I do not know,” that is good. But the calamity is if he says: “It was not reported from the Messenger of Allah.” It is not permissible to use the phrase “It is not reported.” Even if he means within the limit of his knowledge, he should use “I do not know” or “I do not know that” or “I have not come across anything on this matter” or the like.