The earliest external testimony, quoted from public-domain translations: Papias (via Eusebius, Hist. Eccl. 3.39.15–16, NPNF): "Mark having become the interpreter of Peter, wrote down accurately whatsoever he remembered… Matthew put together the oracles in the Hebrew language." Irenaeus (Against Heresies 3.1.1): Matthew among the Hebrews; Mark, "the disciple and interpreter of Peter"; Luke, "the companion of Paul"; then John at Ephesus.
Clement of Alexandria (via Eusebius 6.14.5–7): the gospels with genealogies first; "last of all John… composed a spiritual gospel." Tatian's Diatessaron (c. 170) harmonized exactly four; the Muratorian Fragment lists Luke third and John fourth.
And Irenaeus' four-creatures argument (3.11.8), with the honest note that his symbol assignments (John = lion, Mark = eagle) differ from Jerome's now-familiar set (Matthew = man, Mark = lion, Luke = ox, John = eagle): the symbols are reception history, not Scripture's own identification (Ezek 1:10; Rev 4:7). Sources: four-gospels brief §§2–3; all works PD (ANF/NPNF, Schaff).