2 Corinthians 8:18
“And we have sent together with him the brother whose praise in the Gospel is spread through all the Churches.”
And who is this brother? Some indeed say, Luke, because of the history which he wrote, but some, Barnabas; for he calls the unwritten preaching also Gospel. And for what cause does he not mention their names; while he both makes Titus known by name, and praises him for his cooperation in the Gospel, (seeing that he was so useful that by reason of his absence even Paul could do nothing great and noble; for, “because I found not Titus my brother, I had no relief for my spirit,”
Source: Homilies on Second Corinthians (New Advent)