Musar על עבודה זרה 35:4

And it was because of the "foot of pride" that the daughter of Rabbi Hanina ben Teradion was punished, for it was decreed that she dwell in a brothel because on one occasion, when she was walking before the great men of Rome, she heard them say, "How beautiful are the steps of this maiden!" Whereupon, she took particular care of her step (Abodah Zarah 18a).

Certainly, it requires great work to attain Shelemut (perfection) in this trait. For the recognized and well known sins are easy to guard from for their evil is apparent. But the scrutiny which is necessary for Cleanliness is the most difficult. For the sin is concealed by rationalizations that it is permitted as I wrote and as the sages of blessed memory said: "the sins which a man treads with his heels surround him at the time of Judgment" (Avodah Zara 18a).

However, that which requires deeper analysis and greater work is in mixtures of the forbidden, namely, sometimes a person does a mitzva really for its own sake, namely, that thus our Father decreed, but he cannot refrain from including with it some other motive, such as that other people praise him or that he receive a reward for it. Sometimes, although he does not actually intend that others praise him, nevertheless in rejoicing on the praise received, he puts more effort to improve it. This is similar to the story of Rabbi Chanina ben Teradyon's daughter who was once walking in a graceful manner. When she heard (the Romans) saying: "how beautiful that girl walks", she immediately tried to become more meticulous [in her gracefulness] (Avodah Zara 18a). Hence, this addition stemmed from the spurring of the praise with which they praised her.