What is the prayer that starts with "Forgive me, Father, for I have sinned"?

Although there is no particular ecclesiastical prayer that begins with that phrase, the line "Forgive me, Father, for I have sinned" is usually spoken at the start of a Roman Catholic confession. This is called the Sacrament of Reconciliation, and it is mandated by the Roman Catholic Church for all Catholics.

After the line, "Forgive me, Father, for I have sinned," the person proceeds to confess their various sins to the priest. The priest generally offers words of encouragement or guidance and then prescribes the penance (what they should do to absolve the them of sin) after announcing that God has forgiven the person for his or her sins.