It's in the screenplay -- a brief mention aboard Spacelab Regula I:
118 - FULL SHOT - IN THE LAB
A moan is heard. All three now converge on the area. As they near it, Saavik's tricorder hums furiously. Be this a room, a locker of some other place, this time they -- "open the door." The limp bodies of Chekov and Terrell fall out.
Admiral Kirk, Enterprise. Come in, please. Come in please.
She will continue sporadically.
Chekov emits a live, reassuring moan.
That's Clark Terrell, Jim, I've served with him.
Vonda McIntyre added a bit more to the novelization, but it originated in the shooting script itself.