Bede's Ecclesiastical History of England, ed. by A.M. Sellar, [1907], at sacred-texts.com
AT this time a great part of the Scots in Ireland, and some also of the Britons in Britain, by the grace of God, adopted the reasonable and ecclesiastical time of keeping Easter. For when Adamnan, priest and abbot of the monks that were in the island of Hii, was sent by his nation on a mission to Aldfrid, king of the English, he abode some time in that province, and saw the canonical rites of the Church. Moreover, he was earnestly admonished by many of the more learned sort, not to presume to live contrary to the universal custom of the Church, either in regard to the observance of Easter, or any other ordinances whatsoever, with those few followers of his dwelling in the farthest corner of the world.
Wherefore he so changed his mind, that he readily preferred those things which he had seen and heard in the English churches, to the customs which he and his people had hitherto followed. For he was a good and wise man, and excellently instructed in knowledge of the Scriptures. Returning home, he endeavoured to bring his own people that were in Hii, or that were subject to that monastery, into the way of truth, which he had embraced with all his heart; but he could not prevail. He sailed over into Ireland, and preaching to those people, and with sober words of exhortation making known to them the lawful time of Easter, he brought back many of them, and almost all that were free from the dominion of those of Hii, from the error of their fathers to the Catholic unity, and taught them to keep the lawful time of Easter.
Returning to his island, after having celebrated the canonical Easter in Ireland, he was instant in preaching the Catholic observance of the season of Easter in his monastery, yet without being able to achieve his end; and it so happened that he departed this life before the next year came round, the Divine goodness so ordaining it, that as he was a great lover of peace and unity, he should be taken away to everlasting life before he should be obliged, on the return of the season of Easter, to be at greater variance with those that would not follow him into the truth.
This same man wrote a book concerning the holy places, of great profit to many readers; his authority was the teaching and dictation of Arculf, a bishop of Gaul, who had gone to Jerusalem for the sake of the holy places; and having wandered over all the Promised Land, travelled also to Damascus, Constantinople, Alexandria, and many islands in the sea, and returning home by ship, was cast upon the western coast of Britain by a great tempest. After many adventures he came to the aforesaid servant of Christ, Adamnan, and being found to be learned in the Scriptures, and acquainted with the holy places, was most gladly received by him and gladly heard, insomuch that whatsoever he said that he had seen worthy of remembrance in the holy places, Adamnan straightway set himself to commit to writing. Thus he composed a work, as I have said, profitable to many, and chiefly to those who, being far removed from those places where the patriarchs and Apostles lived, know no more of them than what they have learnt by reading. Adamnan presented this book to King Aldfrid, and through his bounty it came to be read by lesser persons. The writer thereof was also rewarded by him with many gifts and sent back into his country. I believe it will be of advantage to our readers if we collect some passages from his writings, and insert them in this our History.