Check the AIM.
I'd look in the AIM.
There are a couple issues here.
First, all airplanes are expected to climb and descend at their optimal rate. If they can't climb or descend at least 500 fpm the PIC is required to inform ATC.
Second, you're only required to climb and descend at your optimal rate until within 1000 feet of your assigned altitude. Then you're expected to do so at 1000 fpm. This is to prevent nuisance TCAS warnings. As you may already know the TCAS computer looks at rates of closure. If there's an airplane in cruise at FL250 and you're assigned to climb and maintain FL240 but are zooming up there at 4000 fpm obviously both aircraft are going to get a TCAS warning.
Hope that helps. Sorry I don't have the pertinent paragraph.