After the martyrdom of Paul and of Peter, Linus was the first to obtain the episcopate of the church at Rome. Paul mentions him, when writing to Timothy from Rome, in the salutation at the end of the epistle.
Historia Ecclesiastica
Book III, Chapter 2. The First Ruler of the Church of Rome
1 section
Eusebius of Caesarea
Trans. Arthur Cushman McGiffert, A Select Library of Nicene and Post-Nicene Fathers of the Christian Church, Second Series, Volume I, 1890. Public domain.