There were literally hundreds of 'sayings' attributed to Jesus outside the 4 Gospel tradition. Many of these sayings had narrow sectarian acceptance while one 'saying' especially had near universal appeal.
"Be ye skillful Moneychangers."
This is the most commonly quoted of all traditional sayings. Researchers have found around 70 examples of this saying in extant writings.
There are, as to be expected, variations of this saying. The above wording is from Clement of Alexandria. Here is the larger quote:
"Rightly, therefore, the Scripture in its desire to make us such dialecticians, exhorts us: "Be ye skilful moneychangers," rejecting some things, but retaining what is good". He attributes the quote as from Peter who is quoting Jesus. "If any of you wants to be saved, be competent money-changers". (Clem. Alex., Stroniata)
What follows is drawn from research from sources cited below:
In addition to the sheer number of citations collected by Resch, the references to the saying are spread out across hundreds of years and many distinctive cultural locales. Nevertheless, although the saying performs slightly varying functions in these specific instances, it is always utilized for decidedly similar purposes--in discussions of legitimate authority, accurate discernment, and provenance. The money-changer saying, which may even have been part of an early Christian ordination ritual, provides a metaphor--that is, a guiding model--for determining who is the true successor of Christ and the apostles, the rightfully sanctioned interpreter of Scripture, and the defender against heresy....
In our earliest sources, we find the money-changer saying used in the literature of very different groups of "Christians" The saying was deployed not only in late-second- and third-century C.E. settings to legitimize the ability of bishops who would later be considered "orthodox" (e.g., Clement of Alexandria) to make authoritative religious judgments (R 1, 41-43; see also its use in the Didascalia 2.36 and Apostolic Constitutions [R 21, 52]) but also in texts such as the Coptic Askew Codex, commonly titled Pistis Sophia 134 (R 15a), where Mary Magdalene is directed by Jesus to be an "approved money changer" Epiphanius (Pan. 44.2.7-8 [R 23]) explicitly places the saying in the mouth of Appelles, a disciple of Marcion, who cited it in reference to the ability to judge which passages of Scripture were derived from the "higher God" and which were from the flawed demiurge responsible for the creation of the world. (32) Indicative of its wide acceptance as an undisputed saying of "the Lord" is its use by theological opponents such as Jerome (R 30, 60-62) and Origen (R 2, 3, 27-29, 44-51), who cite it (as do more than a dozen other church fathers) without raising any suspicions about heretical origins. (33) The money changer saying is not surprisingly introduced into discussions of whether knowledge garnered from noncanonical (R 12a) and even pagan philosophical sources (R 32) was "true." Resch's list, which for the most part contains citations from documents composed by fourth- and fifth-century C.E. Christian leaders and scholars generally considered orthodox--for example, Cyril of Jerusalem (R 5), Cyril of Alexandria (R 10, 34-36), Basil the Great (R 6-8), John Chrysostom (R 13, 31, 59), and John Cassian (R 24-25, 63-64)--also includes passages derived from eighth(R 12a, 67), ninth- (R 68), and even fourteenth-century C.E. (R 12b, 16) sources. The money-changer saying is, moreover, encountered in the writings of authors living in widely varying geographical and cultural locations, from Alexandria, Jerusalem, and Antioch to Constantinople and Milan.....
....in early Christian literature the money-changer saying is almost exclusively introduced into discussions concerning authentic "Christianity," on critical issues as important as apostolic succession, canon, and the rooting out of heresy. The saying, for example, is referred to, though not cited by Resch, in the context of events surrounding the ordination of Zacchaeus in the Pseudo-Clementine Homilies. In Hom. 3.60-61, the candidate for the appointment as Peter's successor--as in the case of Jesus' disciple Mary Magdalene in Pistis Sophia 134 (R 23)--must be able and willing to determine Jesus' words "as silver and money are proved among the money changers." Even more startling, in another reference to approved money changers not listed by Resch, Origen argues that it is the true church's "approved money changers" who chose which of the many proliferating gospels and traditions linked to Jesus were deemed legitimate (Hom. on Luke 1.1-4). "Approved money changers" are thereby explicitly linked with the highest authorized leaders of the earliest Christian churches responsible for determining which sayings of and teachings concerning Jesus were authentic. The money-changer agraphon, though it is not a part of the Gospels that would become part of the canon, represents symbolically the judgment of those who determined what was in or out! In the Pseudo-Clementine Homilies 2.51 (R 18), the saying is likewise called upon when judging which sayings from the Jewish Scriptures are true and which are counterfeit. In the proto-orthodox argumentation of both Clement of Alexandria and Origen, the interpretation of Scripture and doctrine is the backdrop for the entrance of the saying.
The importance of this agraphon does not depend only on the number of its appearances, but also on its content, which proved extremely flexible and thus useful in the first centuries when developing Christian communities underwent a period of rapid and significant change and consolidation, mainly brought about through a sharp and not uncontroversial categorization of what was canonical and what apocryphal, what was orthodox and what heretic, what was authoritative and inspired and what was poisonous and evil.
https://opensiuc.lib.siu.edu/cgi/viewcontent.cgi?article=5071&context=ocj
(Alfred Resch, Agrapha: Ausserkanonische Evangelien-Fragjnente in moglichster Vollstdndigkeit zusammengestellt und quellenkritisch untersucht(1906))
As mentioned, this saying, used to justify the various and often contradictory religious leaders' determinations of authenticity and canon, is rather ironically, not found in any known Gospel material. The likely source:
Philo Special Laws IV (77) ....and punishment is due to the unrighteous just as honors should be confirmed to the just, so that no wicked man who is in difficulties, and who conceals the truth, ought to escape punishment through the pity excited by his poverty, since he has done what deserves not pity (how should it?) but great anger. And let the man who undertakes the duty of a judge, like a skillful money-changer, divide and distinguish between the natures of things, in order that confusion may not be caused by the mixing together of what is good with what is spurious.