The earliest non-Christian evidence about Christian hymn singing claims:

They met on a fixed day and sang a hymn to Christ as if a God. (Plin. Y. Ep. 10.96.7)

The “fixed day” was probably Sunday, and the letter from the governor of Bithynia, Pliny the Younger, to the emperor Trajan, dated around year 112 CE, reveals that Christians began writing hymns early on. However,…