Bede's Ecclesiastical History of England, ed. by A.M. Sellar, [1907], at sacred-texts.com
AT this time, the kingdom of the East Angles, after the death of Earpwald, the successor of Redwald, was governed by his brother Sigbert, a good and religious man, who some time before had been baptized in Gaul, whilst he lived in banishment, a fugitive from the enmity of Redwald. When he returned home, as soon as- he ascended the throne, being desirous to imitate the good institutions which he had seen in Gaul, he founded a school wherein boys should be taught letters, and was assisted therein by Bishop Felix, who came to him from Kent, and who furnished them with masters and teachers after the manner of the people of Kent.
This king became so great a lover of the ‘heavenly kingdom, that at last, quitting the affairs of his kingdom, and committing them to his kinsman Ecgric, who before had a share in that kingdom, he entered a monastery, which he had built for himself, and having received the tonsure, applied himself rather to do battle for a heavenly throne. A long time after this, it happened that the nation of the Mercians, under King Penda, made war on the East Angles; who finding themselves no match for their enemy, entreated Sigbert to go with them to battle, to encourage the soldiers. He was unwilling and refused, upon which they drew him against his will out of the monastery, and carried him to the army, hoping that the soldiers would be less afraid and less disposed to flee in the presence of one who had formerly been an active and distinguished commander. But he, still mindful of his profession, surrounded, as he was, by a royal army, would carry nothing in his hand but a wand, and was killed with King Ecgric; and the pagans pressing on, all their army was either slanghtered or dispersed.
They were succeeded in the kingdom by Anna,the son of Eni, of the blood royal, a good man, and the father of good children, of whom, in the proper place, we shall speak hereafter. He also was afterwards slain like his predecessors by the same pagan chief of the Mercians.