S. Gregorii Magni Vita, Auctore Paulo Diacono, in Patrologia Latina, ed. by J.-P. Migne (vol. 75), ca. 757.

Denique a fideli et religioso viro, ac huic nostro Patri sanctissimo pro suæ religionis et utilitatis merito valde familiarissimo, fideliter post obitum ejus nobis narratum didicimus, quod cum idem vas electionis et habitaculum sancti Spiritus visionem ultimam prophetæ Ezechielis interpretaretur, oppansum velum inter ipsum et eumdem exceptorem tractatus sui, illo per intervalla prolixius reticiente, idem minister ejus stylo perforaverit, et eventu per foramen conspiciens, vidit columbam nive candidiorem super ejus caput sedentem, rostrumque ipsius ori diu tenere appositum: quæ cum se ab ore ejusdem amoveret, incipiebat sanctus pontifex loqui, et a notario graphium ceris imprimi. Cum vero reticebat sancti Spiritus organum, minister ejus oculum foramini iterum applicabat, eumque ac sic in oratione levatis ad cœlum manibus simul et oculis, columbæ rostrum more solito conspicabatur ore suscipere.

The iconographic attribute of Gregory I or Gregory the Great is the dove: it is said that while he was dictating his homilies on Ezekiel to his secretary, the latter peered through the curtain that separated them and saw a dove perched on Gregory’s shoulder with its beak between his lips. When the dove removed its beak, the Pope spoke and the secretary wrote down his words, but when Gregory fell silent, the servant peered again and saw the dove placing its beak between his lips once more.