Tradition
St. Cyprian
The Epistles of Cyprian
Letter 56
To Cornelius in Exile, Concerning His Confession.
Cyprian Praises in Cornelius and His People Their Confession of the Name of Christ Even to Banishment; And Exhorts Them to Constancy and to Mutual Prayer for One Another, as Well in Respect of the Approaching Day of Struggle in This Life, as After Death.
Source: The Epistles of Cyprian (New Advent)