Psalm 50: the liturgical psalm of repentance

Have mercy on me, O God; according to the multitude of Thy tender mercies, blot out my transgressions. Wash me thoroughly from mine iniquity and cleanse me from my sin, for I acknowledge my transgression, and my sin is ever before me. Against Thee, Thee only have I sinned and done this evil in Thy sight, that Thou mightest be justified when Thou speakest and prevail when Thou art judged. Behold, I was brought forth in iniquities and in sin did my mother bear me. But behold, Thou desirest truth in my innermost parts, and in my hidden parts Thou shalt make me to understand wisdom.

Purge me with hyssop, and I shall be clean; wash me, and I shall be whiter than the snow. Make me to hear joy and gladness, that the bones which Thou hast broken may rejoice. Turn Thy face from my sin and put out all mine iniquities far from me. Create in me a clean heart, O God, and renew a right spirit within me. Cast me not away from Thy presence, and take not Thy Holy Spirit from me. Restore unto me the joy of Thy salvation, and uphold me with Thy steadfast Spirit. Then shall I teach transgressors Thy ways, and sinners shall be converted unto Thee. Deliver me from blood-guiltiness, O God, the God of my salvation, and my tongue shall sing aloud of Thy righteousness. Open my lips, and my mouth shall show forth Thy praise.

For shouldest Thou desire sacrifice, I would give it Thee; but Thou delightest not in burnt offering. The sacrifice of God is a broken spirit, a broken and contrite heart -- these, O God, shalt Thou not despise.

Do good in Thy good pleasure to Sion; buildest Thou up the walls of Jerusalem. Then shalt Thou be pleased with the sacrifice of righteousness, with oblation and whole-burnt offerings. Then shall they offer young bullocks upon Thine altar.

source: http://www.monachos.net/content/liturgics/liturgical-texts/110-psalm-50-the-liturgical-psalm-of-repentance