Musar su Salmi 18:30

This is what David himself said later in his words: "Reward me, O G-d, according to my righteousness, according to the purity of my hands repay me" (ibid 18:21) and "the L-rd has recompensed me according to my righteousness, according to the purity of my hands before His eyes" (ibid 18:25). This refers to this purity and cleanliness which we have mentioned. He then further said: "For by You I run upon a troop... I have pursued my enemies and overtaken them" (Tehilim 18:30). And he himself further stated: "Who will ascend upon the mountain of G-d, who will stand in His Holy place? He who has clean hands and a pure heart." (Tehilim 24:3).

For it is not only the explicit and well known deed of theft or oppression which is forbidden but rather anything which may eventually lead to such a deed and cause it is included in the prohibition. On this our sages of blessed memory said (Sanhedrin 81a): "'he did not defile the wife of his fellow' (Yechezkel 18:6) - that he did not encroach upon his fellow's trade".

Another way of explaining our Psalm may be that David felt the need to apologize for taking credit for his humility, i.e. he apologized שלא גבה לבו, for having his humility become common knowledge. This in itself would rate as a form of boastfulness. He apologized, for not having remained silent, i.e. אם לא שויתי ודממתי נפשי. Whichever may be the case, the subject of this Psalm is the extreme importance of practicing humility.