Musar על חולין 187:5

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).