Just backing up Alan's post with some screenshot captures from the Dual Wire CAN bus spec. Seems like every couple months there is a post asking where these numbers come from - a good question.
Using two term resistors that matches this spec...
www.ctr-electronics.com/downloads/images/canbus/termRes.png
...to match the typical characteristic impedance of twisted pair...
www.ctr-electronics.com/downloads/images/canbus/physMedSpec.png
...gives us a termination at each end that matches the bus impedance. This gives us the sufficiently good signal quality by reducing signal reflections.
So to wrap up: stick to 120Ω if you are terminating the bus yourself. One end of the resistor is common to green (CANL) and the other end to yellow(CANH). The resistor should be physically at the end of the bus.