The Catholic Encyclopedia cites that it is in a Letter to Marinus. FYI, St. Maximus has several letters addressed to Marinus, and more than one regards the monothelite controversy. I don’t know if the excerpt that brother Vico quoted is from the same letter that contains the defense of Pope Honorius, so it would be dubious for you to base your conclusions on that.
But Vico did provide a footnote from Dom Chapman’s book that states that the specific Letter to Marinus at issue mentions the report of Pope Honorius’ scribe. Dom Chapman gives the reference to the letter, contained in Mansi x. 695. If you can read Latin, have a field day.
I. Maximus The Confessor, Andrew Louth, Routledge, May 22, 1996 - 230 pages
In that book, p. 48, has: "Maximus owes his title ‘Confessor’ to his defence of the Orthodox doctrine of the Person of Christ,against the theological view, emanating from theological circles in Constantinople, and endorsed by imperial authority, that suggested language of one activity, or one will, in Christ, as a compromise with the Monophysites. It is, however, a striking fact that it is with apparent reluctance that Maximus becomes involved in this controversy. Although he follows Sophronius’ lead in rejecting the Alexandrian Pact of Union of 633, to begin with he abides by the Psephos of Patriarch Sergius, defending it as implicitly condemning the Alexandrian Pact-which seems somewhat disingenuous. It is only from 640 that he explicitly attacks Monothelitism, and even then he seems anxious to defend Pope Honorius, the originator of the Monothelite formula, from any personal charge of heresy. (1)
(1) Maximi Confessoris Opuscula theologica et polemica 20. Patrologia Graeca 91:237CD, 244C–245A; 28:328C–329B
syriac.ca/Library/Still%20to%20be%20added/from%20fr%20ken/Maximus%20the%20Confessor.pdf
scribd.com/doc/58362079/Andre-Louth-Maximus-the-Confessor
II. And from Life of St. Maximus, Paul Jungwirth, 13th March 2006, Bibliography, are given some useful sources: Opuscula Theologica et Polemica (Theological and Polemical Minor Works).
Twenty-seven short works from throughout Maximus’ career, most of them falling in the crucial period of the 630s and 640s when monoenergism grew into monothelitism. These are important for understanding Maximus’ christology.
CE: Migne. PG 91.9–285.
CE: C. Steel and B. Markesinis, unpublished.
T: “Opuscula 3 and 7.” Trans. Louth, Maximus the Confessor. Early Church Fathers. New York:Routledge, 1996. Based on Steel and Markesinis. 9
T: “Opusculum 6.” On the Cosmic Mystery of Jesus Christ: Selected Writings from St. Maximusthe Confessor. Trans. Paul M. Blowers. Crestwood, NY: St. Vladimir’s Seminary, 2003. Basedon Migne.
T: The Theological and Polemical Works of Our Father among the Saints, Maximus the Confessor.Trans. Bishop Photios (Joseph P.) Farrell. Catoosa, OK: Seven Councils, 2001. Based on Migne.Unpublished, of doubtful quality.
Key: critical editions (CE) and translations (T)
scribd.com/doc/94115249/St-Maximus-Bibliography