Icahn quickly called in to the show and struck back.
"Ackman is a liar," he said. "He's got one of the worst reputations on Wall Street."
Icahn also accused Ackman of destroying companies by publicly shorting their stocks.
"I've really about had it with this guy," he said. "He's like the crybaby in the schoolyard."
Read MoreAckman: Icahn Wanted to Be My Friend, I Said No
On Friday, Icahn, who has criticized Ackman's much-publicized short in Herbalife, issued a written statement.
"To get the record straight, I never asked Ackman to be my friend," Icahn wrote. "Quite to the contrary, Ackman has stated to me on more than one occasion that it's a shame we are not friends because then he could have invested with me. But, even if we were friends, I would never have invested with him because I believe he takes inordinate risks. HLF I believe proves this point.
Read MoreAckman Defends Calling Herbalife a 'Pyramid Scheme'
"As it has been pointed out HLF can turn out to be 'the mother of all short squeezes.' Selling short 20% of the shares of a company such as HLF with limited partners' money that can be withdrawn, in my opinion, leaves much to be questioned.
"Bill Ackman has recently stated, 'Carl Icahn is a great investor.' I thank him but unfortunately I cannot return the compliment."
Trader disclosure: On Jan. 25, 2013, the following stocks and commodities mentioned or intended to be mentioned on CNBC's "Fast Money" were owned by the "Fast Money" traders: Jon Najarian is long SYMC; Jon Najarian is long MSFT; Jon Najarian is long NFLX; Jon Najarian is long GOOG; Jon Najarian is long KLAC; Jon Najarian is long AMAT; Steve Weiss is long C; Steve Weiss is long TBF; Steve Weiss is long BAC; Steve Weiss is long QCOM.