Felix III (IV) was the bishop of Rome between 526 and 530. The text of his letter is preserved in the Liber Pontificalis Ecclesiae Ravennatensis by Agnellus of Ravenna.

"Agnellus of Ravenna (died ca 846) was a deacon of the cathedral in Ravenna and – by hereditary right - abbot of two monasteries in Ravenna. He wrote his Liber Pontificalis Ecclesiae Ravennatis between 830 and 846, following the model of the Roman Liber Pontificalis. This work provides biographies of all the bishops of Ravenna – from the legendary founder bishop Apollinaris to those active in Agnellus’ own day – and was originally composed to be delivered orally, most likely to clerics of Ravenna. This text is preserved in two manuscripts: one from the fifteenth century (Bibliotec Estense Cod. Lat. 371 X.P.4.9.) and a fragmentary manuscript from the sixteenth century (MS Vat. Lat. 5834). Agnellus bases his account of the lives of late antique bishops on documents preserved in Ravenna, stories which had been transmitted orally and his own experiences in the architectural landscape of ninth-century Ravenna." (Frances Trzeciak, Cult of Saints, E05782 - http://csla.history.ox.ac.uk/record.php?recid=E05782)