You must not curse a deaf person. You must not place an obstacle before a blind person. You must fear your G-d; I am G-d.
Lev. 19:14
Rashi informs us that the instruction not to place an obstacle before a blind person is not meant to be taken literally, for if it were, it would be superfluous: the Torah has previously prohibited us to cause damage to our fellow. It also cannot be meant to refer to giving bad advice (“placing an obstacle”) to someone who is “blind” and ignorant in a certain area, since that, too, is covered in the prohibition against causing damage.
Rather, this verse forbids giving sound advice with ulterior motives. For example, we may not tell someone to sell his field and buy a donkey so that we can then buy his field. Even if this advice is not disadvantageous for the “blind” person, it is still prohibited on account of the motivation of the advisor.
The Torah here teaches us the proper approach toward helping others. In order to optimally benefit another person, we must rid ourselves of any self-oriented motivation, focusing instead solely on the needs of the other person.
— Daily Wisdom 3