Jamieson, Fausset and Brown on Acts 17:15

15. Silas and Timotheus to come to him with all speed--He probably wished their company and aid in addressing himself to so new and great a sphere as Athens. Accordingly it is added that he "waited for them" there, as if unwilling to do anything till they came. That they did come, there is no good reason to doubt (as some excellent critics do). For though Paul himself says to the Thessalonians that he "thought it good to be left at Athens alone" (1Th 3:1), he immediately adds that he "sent Timotheus to establish and comfort them" (Ac 17:2); meaning, surely, that he despatched him from Athens back to Thessalonica. He had indeed sent for him to Athens; but, probably, when it appeared that little fruit was to be reaped there, while Thessalonica was in too interesting a state to be left uncherished, he seems to have thought it better to send him back again. (The other explanations which have been suggested seem less satisfactory). Timotheus rejoined the apostle at Corinth (Ac 18:5).