Sanhedrin 22a: No one may see the king when he is taking a haircut, when he is naked or when he is in the bathhouse, as the Torah says, “You shall surely place upon yourself a king” – his fear must be upon you.
סנהדרין כב. ואין רואין אותו כשהוא מסתפר, ולא כשהוא ערום, ולא כשהוא בבית המרחץ, שנאמר (דברים י״ז) שום תשים עליך מלך ־ שתהא אימתו עליך.
In Lublin, there was a Yid who would go to the bathhouse every Friday and distribute combs to all the people there. When the Lubliner Rav zt”l heard about it, he said, “I want to have a part in such a great mitzvah.”
A certain rav heard about this and laughed it off. He quoted a Gemara, proving that there is no mitzvah in this. The Gemara (Menachos 43b) states that Dovid Hamelech went to the bathhouse and said, “Woe unto me, that I stand without mitzvos.” If distributing combs would be a mitzvah, Dovid Hamelech could have done so and acquired many mitzvos in the process…
When the Lubliner Rav heard this comment, he said, “The Rav should forgive me, but it seems he forgot an explicit Mishnah, in which it says that no person may see the king while he is in the bathhouse (Sanhedrin 22a). If so, Dovid Hamelech could not have distributed combs, as there were no other people at the bathhouse while he was there.”