Laudator Temporis Acti

Edward Kennard Rand, Founders of the Middle Ages (Cambridge: Harvard University Press, 1928; rpt. New York: Dover Press, 1957), p. 78 (on St. Ambrose, footnote omitted):

He is a modest teacher. "When I was rushed from the bench of justice into the priesthood," he says, "I began to teach what I had not learned myself.—The result is that I now must learn and teach at the same time." This confession of Ambrose's must be made by any teacher of any subject at any stage of his career. Sometimes, at the outset, one makes the confession with a certain glee, as if it involved a kind of crime against society which one committed without detection. Later, one perceives that it is the normal condition of the teacher and the vitality of his art.

Ambrose, De Officiis Ministrorum 1.4:

ego enim raptus de tribunalibus atque administrationis infulis ad sacerdotium, docere vos coepi quod ipse non didici. itaque factum est, ut prius docere inciperem, quam discere. discendum igitur mihi simul et docendum est, quoniam non vacavit ante discere.

A. Otto, Die Sprichwörter und sprichwörtlichen Redensarten der Römer (Leipzig: Teubner, 1890), p. 118 (no. 563), also cites Seneca, Ep. 7.8 (homines dum docent, discunt) and Serg. Explan. in Donat. 4.486.11 (cum enim docemus, discimus).