I guess this is what Jay was referring to:
"Main Source soon set off on tour, along with the UMCs and Jaz. One particular night of that tour came up recently in the very public catfight between Jay-Z (then Jaz’s hypeman) and Nas (who rolled with Main Source). During a show in Washington D.C. the sound system cut out, forcing the artists to do the same. The angry crowd rushed the stage and chased the rappers back unto their tour bus.Never knew the story behind this one but always wondered. If you aren't familiar with the quote, it's from "The Takeover" (which has its own wikipedia page by the way lol).
'We were figuring we were gonna have to knuckle down,' explains LP of the incident, which Jay has used to attack Nas. 'Jay came outta nowhere, reachin’ in his gym bag like, ‘Don’t even sweat these niggas. I got that.’ He didn’t show [Nas] the gun like, ‘I’m gonna shoot you,’ but we were all just like, ‘Whew!’' (Source: XXL Mag, 2002)