2 Corinthians 8:18

And we have sent together with him the brother whose praise in the gospel is spread through all the churches [Baynes, in his "Horæ Lucanæ," argues very conclusively that this was Luke. He was at Philippi about this time, and was among those who accompanied Paul from Macedonia (or perhaps Corinth) to Jerusalem (Acts 20:2-6). The phrase "In the gospel" can hardly be taken as indicating that at this time Luke had written his Gospel, but the Gospel which he wrote is evidently not the work of a day. No doubt at this time Luke was so versed in the gospel history as to be fittingly described by the words here used by Paul];