HarperCollins Study Bible Introduction to 1 Thessalonians.
Edgar M. Krentz, The HarperCollins Study Bible: Fully Revised and Updated, edited by Harold W. Attridge (San Francisco: HarperCollins Publishers, 2006), 2005
When Timothy joined Paul in Athens, Paul immediately sent him back to Thessalonica or information about the church (1 Thes. 3:1-2) and then went on to Corinth, where Timothy and Silvanus met him later (Acts 18:1-5). Timothy reported favorably about the life of the community (1 Thes. 3:6-7) but also, it seems, brought news of anxiety over Paul's failure to return (1 Thes. 2:17-18).
Timothy's report stimulated Paul to write 1 Thessalonians from Corinth about 51 CE.