Paul's first (preserved) letter to the church in Corinth.
The apostle Paul planted a church in Corinth around 49-50 AD/CE during a stay of eighteen months on his second "missionary journey". It is clear from the contents of 1 Corinthians that Paul had previously written a letter to this church:
I wrote to you in my letter not to associate with sexually immoral people... (NIV; 1 Corinthians 5:9)
No trace of this letter remains, unless it might be represented by the "fragment" in 2 Corinthians 6:14 - 7:1 as has long been surmised.
1 Corinthians itself shows signs of being a response to reports to Paul, whether in verbal reports about the church (see 1 Cor 1:11), or in writing from the Corinthian Christians themselves (so 1 Cor 7:1).