Musar zu Jeschijahu 54:17

Another practice that interferes with the acceptability of a person’s holy endeavors is the donning of two garments together, namely, donning one of them when the other is already inside it. According to an oral tradition handed down to us from the disciples of the Ari (Pri Eitz Chayim, Sha’ar HaTefillah, Ch. 2) this practice is very injurious to a person, for it allows the Sitrah Acharah a certain degree of access to him. This is especially true regarding the donning of one’s Sabbath clothes, which require extra care, as we have already mentioned. For the Sitrah Acharah surrounds a person striving to cling to him, especially when he is dressed in his Shabbos clothing. Therefore the Ari advised that as a person dons them he should recite the verse, “No weapon that is fashioned will succeed against you and every tongue that rises up against you in judgment will you condemn. This is the inheritance of the servants of Hashem and the recompense from Me for their righteousness, says Hashem” (Yeshaya 54:17). It is also appropriate to recite this verse as one dons one’s outer garment to pray. One should thoroughly examine one’s garments at other times, too, and not only when one is about to pray, because sanctity does not devolve upon any place that is repulsive or soiled.