Musar zu Mischlej 11:9

The third category is he who flatters a wicked man to his face and says to him, "What a charming and good man you are." Now even though this flatterer does not praise the wicked man in public, so that the wicked man will not become a stumbling block to the multitude, still this type of flatterer is guilty of a great sin as it is written, "With his mouth the impious man destroyeth his neighbour; but through knowledge shall the righteous be delivered" (Prov. 11:9). For when he praises him, the wicked man believes him and considers himself good, and his heart will be uplifted and he will be proud and not repent. For a man who is not righteous will say in his heart when people praise him. "I always knew that this was so." And thus the wicked man becomes more corrupt through the flatterer's flattery.