Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Synods of Sardica, Carthage, Constantinople, and Carthage Under St. Cyprian, Which Canons Were Received by the Council in Trullo and Ratified by II. Nice.: Canon CXV

Canon CXV.  (Greek cxvi.)

That in the Lord’s Prayer the Saints say for themselves:  “Forgive us our trespasses.”

It has seemed good that whoever should say that when in the Lord’s prayer, the saints say, “forgive us our trespasses,” they say this not for themselves, because they have no need of this petition, but for the rest who are sinners of the people; and that p. 499 therefore no one of the saints can say, “Forgive me my trespasses,” but “Forgive us our trespasses;” so that the just is understood to seek this for others rather than for himself; let him be anathema.  For holy and just was the Apostle James, when he said, “For in many things we offend all.”  For why was it added “all,” unless that this sentence might agree also with the psalm, where we read, “Enter not into judgment with thy servant, O Lord, for in thy sight shall no man living be justified;” and in the prayer of the most wise Solomon:  “There is no man that sinneth not;” and in the book of the holy Job:  “He sealeth in the hand of every man, that every man may know his own infirmity;” wherefore even the holy and just Daniel when in prayer said several times:  “We have sinned, we have done iniquity,” and other things which there truly and humbly he confessed; nor let it be thought (as some have thought) that this was said not of his own but rather of the people’s sins, for he said further on:  “When I shall pray and confess my sins and the sins of my people to the Lord my God;” he did not wish to say our sins, but he said the sins of his people and his own sins, since he as a prophet foresaw that those who were to come would thus misunderstand his words.

Notes.

Ancient Epitome of Canon CXV.

Whoso expounds this, “forgive us our trespasses” as speaking only of the multitude and not of individuals let him be anathema:  Since Daniel even he can behold saying with the multitude “I confessed my sins and the sins of my people.”

This is Canon vij. of Carthage, a.d. 418.