officially, Never Scared (Remix). it evolved from there once Jada did his 2004 Kiss of Death album and more features after 2003 but there was a story that Jadakiss told that I can't find anymore. Jadakiss used to do his "whoo, haha" as vocal exercises to clear his throat and to enunciate clearly when he starts rapping. the engineer recorded it, thought it was dope, put the Jadakiss laugh as the intro of Never Scared remix, the people liked it, and jada converted it into his signature adlib...the rest is history.
50 Cent did the "aha" laugh at the end of Wanksta back in 2002, and Jadakiss told his story back when he and 50 had beef to dismiss crediting 50, so who knows who started it first