[updated see below] How do we know that lashon harah (making derogatory statements about others) is prohibited? The most obvious candidate for the prohibition of lashon is Vayikra (19:16), Do not spread gossip amongst your people.... However the Rambam in Sefer HaMtizvos (301) says that this is a prohibition for rechilus (gossip) and motzi shem rah (slander) and does not mention lashon harah at all. .In contrast the Rambam in his later work Mishna Torah (Hilchos De'os 7:1) states that this verse is the source for the prohibition of rechilus (gossip) and that lashon hara and motzi shem rah are also included in this Torah commandment. The Chofetz Chaim says that the verse is only about rechilus and that lashon harah is learned by kal v'chomer from rechilus. [He says that consequently there is a problem for the Ravad who disagrees with the Rambam and says that rechilus is more severe than lashon harah – and thus the Ravad must learn lashon harah from a different verse.]
The Bavli also does not provide a verse for the prohibition of lashon harah. Rather the concern is for the prohibition of slander. Kesuvos (26a) mentions a debate regarding motzi shem rav – is it learned from Vayikra (19:16) or is learned from Devarim (23:10) Guard yourself from all evil. It does not ask about lashon harah. The verse in Vayikra is also cited as the source of rules regarding judges. He is not to be harsh to one litigant and gentle to the other. The deliberations of the court are not to be revealed.
One obvious explanation as to the lack of sources is that the Talmud does not clearly differentiate between gossip (rechilus) and derogatory comments (lashon harah) but rather uses the terms interchangably. We see this also from Yerushalmi (Peah 1:1) which does in fact ask for the source of prohibition of lashon harah. It says there is a dispute whether lashon harah is learned from the prohibition of gossip (Vayikra 19:16) or from Guard yourself from all evil (Devarim 23:10) – just as the Bavli asked regarding slander (motzi shem rah). Rav La says that prohibition against spreading gossip indicates a prohibition against "lashon harah –rechilus." [both terms together as if lashon harah is an adjective modifying rechilus] Rav Nechmiah said one should not be like a peddler who carries the words of one and brings them to another and vice versa.
אזהרה ללשון הרע מניין ונשמרת מכל דבר רע אמר רבי לא תני רבי ישמעאל לא תלך רכיל בעמך זו רכילות לשון הרע תני ר' נחמיה שלא תהא כרוכל הזה מטעין דבריו של זה לזה ודבריו של זה לזה
The question is then whether it is true that before the Rishonim there was not a precise differential meaning for rechilus and lashon harah and that the terms were used interchangably? This would make sense if speaking negatively about others was a moral issue rather than a legal one. In other words if speaking lashon harah was a problem of character or midos and not halacha. If this is true than the revolution of the Chofetz Chaim was not that he was the first to create a Shulchan Aruch of the issur of lashon harah but rather that he succeeded in transforming lashon harah from midos to halacha. While it is true that bad midos are also prohibited by halacha – but there is no need for precisely describing the parameters as the Chofetz Chaim has done regarding the prohibitions of lashon harah/rechilus.
This issue of whether lashon harah is primarily midos or issur is discussed by Rav Asher Weiss (Minchas Asher Vayikra 19:16 #41) and Dr. Benny Brown Pdf fixed link [ and Daas Torah link] (From Principles to Rules and from Musar to Halakhah:The Hafetz Hayim’s Rulings on Libel and Gossip) and Rabbi Asher Buchman pdf (Legislating Morality: The Prohibition of Lashon Hara in Hakira) who discusses why Rambam placed these laws in Hilchos De'os which describes character perfection)
update (10/16/13) The Chofetz Chaim himself made the distinction between moral (mussar) and halacha in his explanation of how he could use Rabbeinu Yonah as a source - when it was mussar (moral) not a halacha sefer
Chofetz Chaim (Lashon Harah – Introduction: Comment): The reader should not find it astounding that even though my entire sefer is based on halachic principles and conclusions, but I nevertheless cite in a number of places proofs from Rabbeinu Yonah's sefer – Shaarei Teshuva which is a mussar book [not halacha]. That is because if one examines Rabbeinu Yonah's words in a number of places it is clear that he was very careful with his words and they do not deviate from the halacha. In particular this is true concerning his writings about lashon harah. In fact everything he wrote there is a source in the Talmud as I will explain G‑d willing in this sefer. However he is very sparing in his words and he doesn't cite his sources contary to the practice of Rishonim. Nevertheless, in most cases I did not depend exclusively on the rulings of Rabbeinu Yonah – except in circumstances where a leniency could be inferred (and this is true for other Mussar books).
update (10/16/13) The Chofetz Chaim himself made the distinction between moral (mussar) and halacha in his explanation of how he could use Rabbeinu Yonah as a source - when it was mussar (moral) not a halacha sefer
Chofetz Chaim (Lashon Harah – Introduction: Comment): The reader should not find it astounding that even though my entire sefer is based on halachic principles and conclusions, but I nevertheless cite in a number of places proofs from Rabbeinu Yonah's sefer – Shaarei Teshuva which is a mussar book [not halacha]. That is because if one examines Rabbeinu Yonah's words in a number of places it is clear that he was very careful with his words and they do not deviate from the halacha. In particular this is true concerning his writings about lashon harah. In fact everything he wrote there is a source in the Talmud as I will explain G‑d willing in this sefer. However he is very sparing in his words and he doesn't cite his sources contary to the practice of Rishonim. Nevertheless, in most cases I did not depend exclusively on the rulings of Rabbeinu Yonah – except in circumstances where a leniency could be inferred (and this is true for other Mussar books).