This is an anecdote recorded by Plutarch about the fall of Athens to the Spartan commander Lysander, which ended the Peloponnesian War (431-404 BC). Plutarch says the exchange supposedly went as follows:

One hears it said by the Spartans that Lysander wrote to the ephors: "Athens is taken"; and that the ephors wrote back to Lysander: "'Taken' would have been enough".

-- Plutarch, Life of Lysander 14.4

The supposed message of Lysander, then, was only three words long: ἁλώκαντι ταὶ Ἀθᾶναι, "Athens is taken". The ephors supposedly replied that one word would have been enough: ἑαλώκειν, "taken". There is slight variation in the conjugation of the verb but it renders pretty well in English.

What is interesting about the episode, though, is that Plutach himself did not believe it really happened. You'll note that he says the Spartans themselves like to tell this story, which instantly frames it as untrustworthy. Immediately after he gives the quotes, he adds, "but this story was invented for its neatness' sake." The Spartans liked to boast that they were men of few words, and that they trained their citizens from childhood to say as much as possible with the fewest possible words. There are many other anecdotes connected to this self-promoted stereotype. In many other cases Plutarch is more than happy to repeat such stories, but here he decided to be skeptical. He simply did not believe this exchange was plausible, and set it aside as Spartan propaganda. To prove it, he went on to quote the actual terms dictated by the ephors, which they sent to Lysander to impose upon the defeated Athenians. It is clear that they wrote a lot more than just some pithy phrase.

If we assume that Plutarch was being too critical here, though, we could also explain the quote as a deliberate rebuke of Lysander by the ephors, who were getting concerned about his growing power and influence. Lysander was not a Spartan citizen, but he had proved more effective than any other Spartan commander of the second half of the Peloponnesian War; indeed, he had finally destroyed the Athenian fleet, which decided the outcome. In the process of "liberating" Athens' imperial subjects, he had set up many narrow oligarchies across the Greek world that were more loyal to him personally than they were to their new Spartan overlords. The ephors and kings feared that Lysander, who had no political rights at home (being a non-citizen), might be plotting to overthrow the Spartan system and make himself tyrant. In this instance they may well have chosen to censure him as "not a true Spartan" rather than celebrating his undeniable achievement, just to remind him that he could never be as great as the city that raised him up.