Church History by Eusebius Pamphilius
Section 9. The Death of Eusebius.
In less than two years after the celebration of his tricennalia, on May 22, 337 a.d., the great Constantine breathed his last, in Nicomedia, his former Capital. Eusebius, already an old man, produced a lasting testimonial of his own unbounded affection and admiration for the first Christian emperor, in his Life of Constantine. Soon afterward he followed his imperial friend at the advanced age of nearly, if not quite, eighty years. The exact date of his death is unknown, but it can be fixed approximately. We know from Sozomen (H. E. III.5) that in the summer of 341, when a council was held at Antioch (on the date of the Council, which we are able to fix with great exactness, see Hefele, Conciliengesch. I. p.502 sq.) Acacius, Eusebius' successor, was already bishop of Cæsarea. Socrates (H. E. II.4) and Sozomen (H. E. III.2) both mention the death of Eusebius and place it shortly before the death of Constantine the younger, which took place early in 340 (see Tillemont's Hist. des Emp. IV. p.327 sq.), and after the intrigues had begun which resulted in Athanasius' second banishment. We are thus led to place Eusebius' death late in the year 339, or early in the year 340 (cf. Lightfoot's article, p.318).