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.

EusHE 8.6.1