I found this in Suetonius' biography of Caesar Augustus.
There is also a book of Epigrams, no larger than the last [book], which [Augustus] composed almost entirely while he was in the bath. These are all his poetical compositions: for though he began a tragedy with great zest, becoming dissatisfied with the style, he obliterated the whole; and his friends saying to him, "What is your Ajax doing?"
He answered, "My Ajax met with a sponge."
- Chapter 85