Church History by Eusebius Pamphilius
Chapter VI.--Clement of Alexandria.
Clement having succeeded Pantænus, had charge at that time of the catechetical instruction in Alexandria, so that Origen also, while still a boy, was one of his pupils. In the first book of the work called Stromata, which Clement wrote, he gives a chronological table, bringing events down to the death of Commodus. So it is evident that that work was written during the reign of Severus, whose times we are now recording.