The story is in Macrobius, Saturnalia, book 2, chapter 4:
 As he went down from his residence on the Palatine, a seedy-looking Greek used to offer him a complimentary epigram. This the man did on many occasions without success, and Augustus, seeing him about to do it again, wrote a short epigram in Greek with his own hand and sent it to the fellow as he drew near. The Greek read it and praised it, expressing admiration both in words and by his looks. Then, coming up to the imperial chair, he put his hand in a shabby purse and drew out a few pence, to give them to the emperor, saying as he did so: “I swear by thy Good Fortune, Augustus, if I had more, I should give you more.” There was laughter all round, and Augustus, summoning his steward, ordered him to payout a hundred thousand sesterces to the Greek.