This prayer is called the Act of Contrition. It is to be said by a person before taking the sacrament.

O my God, I am heartily sorry for having offended you. I detest all my sins because of your just punishments, but most of all because they offend you, my God, who are all good and deserving of all my love. I firmly resolve, with the help of your grace, to sin no more and to avoid the occasions of sin. Amen.

This prayer is said by the priest near the end of the sacrament.

God, the Father of mercies through the death and resurrection of his Son has reconciled the world to himself and has sent the Holy Spirit among us for the forgiveness of sins; through the ministry of the Church may God give you pardon and peace, and I absolve you from your sins in the name of the Father, and of the Son and of the Holy Spirit.