As I said, it was in “The Shepherd” by Hermas. That work is not quoted in the list you referenced.
There is commentary following this selection
The Shepherd, by Hermas, brother of Pope St.Pius I, who ruled 140-150.
Mandate 4.3.1-6: Hermas says to the angel of Penance:
“I have heard, sir, from some teachers that there is no other
means of repentance than the one when we went down into the
water and obtained remission of our previous sins. He said to
me, you have heard rightly, for that is true. He who has
received remission of sin should never sin again, but live in
purity. But since you ask carefully about everything, I will
explain this too to you, not to give an excuse to those who in
the future will believe or to those who have already believed in
the Lord. For those who have already believed or are going to
believe have no means of repentance of sins, but have the
remission of their previous sins. For those who were called
before these days, the Lord appointed means of repentance, For
the Lord knows the heart and since He knows in advance
everything, He knew the weakness of man, and the cunning
craftiness of the devil,that he will do some evil to the
servants of God and will deal wickedly with them. The Lord then,
since He is full of compassion had mercy on His creatures, and
established this means of repentance. And to me was given power
over this means of repentance. But I tell you, he said, after
that great and solemn calling, if a man should be tempted by the
devil and sin, he has one means of repentance. But if he sins
repeatedly and repents, it does him no good, for scarcely shall
he live.”
COMMENT: Most scholars think the passage is deliberately obscure, for
psychological reasons.It opens by saying there is only one means of
forgiveness, Baptism. But at once it adds that there is another, but not
all may use it. Implication is that repentance might not be real in many,
especially if they denied Christ in the Roman court, planning to use the
sacrament afterwards. Their repentance then would probably not be sincere -
it was preplanned, and there was no a real change of heart. But after long
and hard penance, there could be a change of heart. So,in Parable 9.16 we
read: “It is impossible for him to be saved who now denies His Lord, but
for those who denied Him long ago, repentance seems possible.” Also, the
angel, speaks of one means - which could imply that the Sacrament of
Penance could be used only once in a lifetime. It says one should never sin
again -this is the Baptismal seal, which marks one as God’s property, and
one should never break the seal. Yet we do see here,in spite of the
deliberate obscurity, that there was a Sacrament of Penance.
We use a regressive method: we go back to the first point at which a
doctrine is clear - earlier are only unclear statements. But since we know
that in the Patristic age any change in doctrine provoked a storm, if there
was no storm at the time of the first clear statement, we gather that the
teaching was around long before, even from the beginning. We know too that
the penetration by the Church into the deposit of revelation grows with
time: so we need not suppose everything was clear at the start, e.g.,the
case of the Immaculate Conception. This all stems from the promise at the
Last Supper, in John 16:13 that He would send the Holy Spirit to lead them
into all truth. It did not mean new revelations, but a deeper penetration.
ewtn.com/library/DOCTRINE/PENANC.TXT
That’s why I gave the quotes I did, to show that confession was not limited to one time.
