Trials and Tribble-ations - S5-E6
Trivia: When the agents ask Sisko which Enterprise ship, he says "the first one." One agent also responds with there's been 6 Enterprises. However, in the 2001 TV series "Star Trek: Enterprise", we learn there was an Enterprise before the "first" one. And (as of 2020), there have been 8, or possibly 9, Enterprise ships. (I have listed this a trivia since at the time of the show it wouldn't have been a mistake. But one could argue this is a mistake or that the TV series ST:E made a mistake, since it's been established TOS Enterprise was the first ship with that name).