Proverbs 15:8. “The sacrifice of the wicked is an abomination to the Lord: but the prayer of the upright is His delight.”
There really isn’t much commentary necessary here. If we regard sin in our hearts, all our sacrifices are like the stench of a dead body in God’s nostrils (that’s what the Hebrew word for abomination means). But when we pray in humility and sincere love for God, He is delighted.
I am reminded of another passage:
9 And He spake this parable unto certain which trusted in themselves that they were righteous, and despised others:
10 Two men went up into the temple to pray; the one a Pharisee, and the other a publican.
11 The Pharisee stood and prayed thus with himself, God, I thank thee, that I am not as other men are, extortioners, unjust, adulterers, or even as this publican.
12 I fast twice in the week, I give tithes of all that I possess.
13 And the publican, standing afar off, would not lift up so much as his eyes unto heaven, but smote upon his breast, saying, God be merciful to me a sinner.
14 I tell you, this man went down to his house justified rather than the other: for every one that exalteth himself shall be abased; and he that humbleth himself shall be exalted.