Rashi on Psalms
And do not take...from my mouth a word of truth Heb. תצל. Do not separate from my mouth a word of truth, as (Gen. 31:9): “And God separated (ויצל) your father’s property and gave it to me.” I found this.
Rashi on Psalms
utterly...a word of truth in all my instructions [regarding ritual law] and in all my legal decisions, that I should not stumble over them.