It is "Pope Gregory and the English Slave Boys," from an Anglo-Saxon version of the Venerable Bede's original Latin version of Book II, Chapter 1 of his Ecclesiastical History of the English People. After my translation, I will include another English translation (not by me) for comparison, and then the Latin original. I was unfortunately unable to easily find an online text of the Anglo-Saxon version from which I made my translation, or I would have included it here also.
In 1988, when I first made my translation, I used the Anglo-Saxon version because Latin at that time was too difficult for me to be able to attempt a translation. I was at that time more familiar with the Anglo-Saxon language. I will not now vouch for the complete accuracy of my translation; indeed, I have not even looked at it for many years, until now, nor compared it with either the Latin original, the Anglo-Saxon gloss, nor even any other English translations. So I presently have really no idea how well or poorly a job I did then. You be the judge.
My own translation (dated Nov. 6, 1988):
It is not for us, then, to try to silence the fame and notoriety of the story which is told us by our elders concerning the blessed Gregory [and his encounter with the English slave boys]. For this cause did men greatly wonder at him, that he displayed such an eager interest in these boys, who were slaves taken from among our own kindred.
They say that on a certain day, there came to Rome new traders from Britain, who also brought with them many and various slaves and chattels for to sell in the markets. There also came at that time many to buy of the things which were brought. It then happened that Pope Gregory (among the many others) also went thither, and chanced to see, among the many other goods and chattels for sale, several beautiful boys--boys with noble, pure hair, and fair and white bodies.
As soon as he saw and beheld them, he immediately stopped and asked where they were from. A man answered and said that they were brought from Britain's island, and also informed the Pope that the rest of the people of that far-off island looked like these particular boys. After which, Pope Gregory inquired whether those same people were Christian--they who, unbeknownst to him, were yet Pagan. A man answered him and said that they were still heathen. At that reply, Pope Gregory swore strongly in his heart, and exclaimed: "Welaway! That is very sad--that such wondrously fair lives and such light and beautiful boys should be the property of the Dark Prince!"
Then he asked what the name of the people was from whom they came. A man answered and said that they were called 'Angles'. Whereupon Pope Gregory immediately remarked: "It is meet that they should be called so, for they certainly have the most angel-like of visages, and also such forms as would befit wards of heaven."
Then he questioned still further, and said: "What are the particular tribe called from whom these boys were taken?" Then answered him a man and said that they were named 'Dere'. Quoth Pope Gregory: "Well it is that they are called Dere--that is: 'de ira eruti' [from wrath saved]; they should be saved from the destruction of God's wrath, and called to Christ's mercy."
Then he went on, and asked what their king was called. And a man answered him and said that he was called 'Alle'. And then the Pope continued his game of sporting with his words by making puns on the names, and said: "Alle--Alleluia! That name is well-befitting a king of such people as these--that thus the praise and glory of God our Creator should be sung in those parts."
(Here follows another English translation for comparison:)
Nor is the account of St. Gregory, which has been handed down to us by the tradition of our ancestors, to be passed by in silence, in relation to his motives for taking such interest in the salvation of our nation. It is reported, that some merchants, having just arrived at Rome on a certain day, exposed many things for sale in the marketplace, and abundance of people resorted thither to buy: Gregory himself went with the rest, and, among other things, some boys were set to sale, their bodies white, their countenances beautiful, and their hair very fine. Having viewed them, he asked, as is said, from what country or nation they were brought? and was told, from the island of Britain, whose inhabitants were of such personal appearance. He again inquired whether those islanders were Christians, or still involved in the errors of paganism? and was informed that they were pagans. Then fetching a deep sigh from the bottom of his heart, "Alas! what pity," said he, "that the author of darkness is possessed of men of such fair countenances; and that being remarkable for such graceful aspects, their minds should be void of inward grace." He therefore again asked, what was the name of that nation? and was answered, that they were called Angles. "Right," said he, for they have an Angelic face, and it becomes such to be co-heirs with the Angels in heaven. What is the name," proceeded he, "of the province from which they are brought?" It was replied, that the natives of that province were called Deiri. "Truly are they De ira," said he, "withdrawn from wrath, and called to the mercy of Christ. How is the king of that province called?" They told him his name was Ælla: and he, alluding to the name said, "Hallelujah, the praise of God the Creator must be sung in those parts."
From ORB's Internet Medieval Sourcebook, Paul Halsall, editor
(at http://www.fordham.edu/halsall/sbook.html )
(And here is Bede's Latin original:)
Nec silentio praetereunda opinio, quae de beato Gregorio traditione maiorum ad nos usque perlata est; qua uidelicet ex causa admonitus tam sedulam erga salutem nostrae gentis curam gesserit. Dicunt, quia die quadam cum, aduenientibus nuper mercatoribus, multa uenalia in forum fuissent conlata, multi ad emendum confluxissent, et ipsum Gregorium inter alios aduenisse, ac uidisse inter alia pueros uenales positos candidi corporis, ac uenusti uultus, capillorum quoque forma egregia. Quos cum aspiceret, interrogauit, ut aiunt, de qua regione uel terra essent adlati. Dictumque est, quia de Brittania insula, cuius incolae talis essent aspectus. Rursus interrogauit, utrum idem insulani Christiani, an paganis adhuc erroribus essent inplicati. Dictum est, quod essent pagani. At ille, intimo ex corde longa trahens suspiria: ‘Heu, pro dolor!’ inquit, ‘quod tam lucidi uultus homines tenebrarum auctor possidet, tantaque gratia frontispicii mentem ab interna gratia uacuam gestat!’ Rursus ergo interrogauit, quod esset uocabulum gentis illius. Responsum est, quod Angli uocarentur. At ille: ‘Bene,’ inquit; ‘nam et angelicam habent faciem, et tales angelorum in caelis decet esse coheredes. Quod habet nomen ipsa prouincia, de qua isti sunt adlati?’ Responsum est, quod Deiri uocarentur idem prouinciales. At ille: ‘Bene,’ inquit, ‘Deiri; de ira eruti, et ad misericordiam Christi uocati. Rex prouinciae illius quomodo appellatur?’ Responsum est, quod Aelli diceretur. At ille adludens ad nomen ait: ‘Alleluia, laudem Dei Creatoris illis in partibus oportet cantari.’
From the web-site http://thelatinlibrary.com/bede.html