I found the complete dialogue from Monty Pyhton and the Holy Grail.
Seems I misremembered it, but IMBD helped me out…
King of Swamp Castle: Guards, make sure the prince doesn’t leave this room until I come and get him.
Guard #1: Not to leave the room… even if you come and get him.
Guard #2: [hiccups]
King of Swamp Castle: No, no. Until I come and get him.
Guard #1: Until you come and get him, we’re not to enter the room.
King of Swamp Castle: No, no, no. You stay in the room, and make sure he doesn’t leave.
Guard #1: And you’ll come and get him.
Guard #2: [hiccups]
King of Swamp Castle: Right.
Guard #1: We don’t need to do anything, apart from just stop him entering the room.
King of Swamp Castle: No, no. Leaving the room.
Guard #1: Leaving the room, yes.
King of Swamp Castle: All right?
Guard #2: [hiccups]
Guard #1: Right. Oh, if, if, if, uh, if, if, uh, if, if, if, we… oh, if… oh…
King of Swamp Castle: Look, it’s quite simple. You just stay here, and make sure he doesn’t leave the room. All right?
Guard #2: [hiccups]
Guard #1: Oh, I remember, uh, can he leave the room with us?
King of Swamp Castle: No, no, no, no, you just keep him in here, and make sure…
Guard #1: Oh yeah, we’ll keep him in here, obviously, but if he had to leave, and we were with him…
King of Swamp Castle: No, just keep him in here…
Guard #1: Until you, or anyone else…
King of Swamp Castle: No, not anyone else. Just me.
Guard #1: Just you.
Guard #2: [hiccups]
King of Swamp Castle: Get back.
Guard #1: Get back.
King of Swamp Castle: All right?
Guard #1: Right, we’ll stay here until you get back.
King of Swamp Castle: And make sure he doesn’t leave.
Guard #1: What?
King of Swamp Castle: Make sure he doesn’t leave.
Guard #1: The prince?
King of Swamp Castle: Yes, make sure he doesn’t leave.
Guard #1: Oh, yes, of course.
[Points at Guard #2]
Guard #1: I thought you meant him. You know, it seemed a bit daft me I were to guard him when he’s a guard.
King of Swamp Castle: Is that clear?
Guard #2: [hiccups]
Guard #1: Oh, quite clear. No problems.
King of Swamp Castle: Right.
[King of Swamp Castle turns to leave the room, both guards follow him]
King of Swamp Castle: Where are you going?
Guard #1: We’re coming with you.
King of Swamp Castle: No, no, no. I want you to stay here and make sure he doesn’t leave.
Guard #1: Oh, I see. Right.