Musar for Chullin 187:30

The seventh category of falsehood is one who deceitfully tells his friend that he has done a favor for him, or that he has spoken well of him, but has not done so. Concerning this our Sages, of blessed memory, said, "It is forbidden to deceive your fellow creatures, even a heathen, for there is in this a sin, inasmuch as we are obliged to speak words of truth, for this is one of the foundations of the soul" (see Hullin 94a).

There is a flattery that is very evil, for example, where a man flatters his companion and speaks to him sweet words, in order that his companion should rely upon him and trust him, and after he trusts him and relies upon him, then this man deceives him, and this is like the matter of which it is said, "For in vain the net is spread in the eyes of any bird; and these lie in wait for their own blood, they lurk for their own lives" (Prov. 1:17—18). The meaning of the verse is that those who trap birds throw wheat upon the trap, and when the birds come to eat the wheat that is spread upon the net, they are captured. And this type of flattery is like such a hunter. The Sages have forbidden us to flatter, and they said (Hullin 94a), A man should not send a gift to his companion when he knows that his companion will not accept it, and he should not invite his companion to eat with him when he knows that he will not eat with him. And if a man wants to open a barrel of wine in order to sell it, and his companion comes to buy wine, he should not say to him, "I want to open a new cask just for you." All these and the like are called "theft of the mind." And our Sages forbade us to flatter or to "steal people's minds" (ibid.).