Musar for Sheviit 10:11

The eighth category of falsehood is he who boasts of qualities that he does not possess. And this is a great sin. And even if he really possesses these qualities when he praises himself it appears from his words that he did not do his charitable or generous acts for the sake of Heaven, but for his own sake, for his own praise. And our Sages, of blessed memory, said, "He whom others honor because they believe that he knows two tractates of the Talmud, and he knows only one tractate, is obligated to say, 'I know only one tractate' " (T.P. Shebi'ith 10:5). All the more so is it forbidden to lie and to boast about qualities that one does not possess at all.