The stock gained about 4 percent in extended trading. Roughly an hour after the market close, Herbalife traded almost quadruple its 30-day average volume of 2.9 million shares.
Herbalife Chairman and CEO Michael Johnson said, "We appreciate the support of all of our investors and are particularly grateful to Carl Icahn and the conviction he shares, and continues to show in our business, as demonstrated by today's significant increase in his stake in the Company."
Icahn said that, last month, Herbalife granted him permission to up his stake to 35 percent. Friday's trade increases Icahn's stake to 20.8 percent from 18.3 percent.
"At the time of the disclosure, Ackman declared that I have no interest in increasing my position in Herbalife. This was obviously another misstatement of the facts by Ackman since today I bought another 2.3 million shares," he said in a statement.
Earlier on Friday, Ackman told CNBC on Friday he was approached indirectly by Icahn to purchase the billionaire's stake in Herbalife — Ackman's longtime short target.
Asked why Icahn would want to sell his stake in Herbalife, Ackman responded: "I think he knows this is toast" and "he's made bunch of money."
Icahn refuted Ackman's allegations. He said that while many investment bankers "frequently make bids for our large positions ... I have never given Jefferies an order to sell any of our Herbalife shares."
Jefferies told CNBC it never received a sell order from Icahn or anyone at his firm.
He continued that while "Ackman may be a smart guy but he has clearly succumbed to the same dangerous (and sometimes fatal) malady that afflicts many investors – he's developed a very bad case of 'Herbalife obsession.'"
Icahn said that obsessing over the value of stock can be "the undoing of many investors" and that Ackman's earlier comments are a "perfect example."
The chairman of Icahn Enterprises reiterated his confidence in Herbalife.
"I continue to believe in Herbalife: it's a great model that creates a great number of jobs for people," he said.
—CNBC's Scott Wapner, Matt Belvedere, Robert Hum and Gina Francolla contributed to this report.