Output resistance of source degenerated amplifier

If we stick a resistor on the source Rs on the above model, what would be the output resistance? I thought you just open up the current sources, so it would be rds in series with Rs, Rout = rds + Rs. But my textbook gives Rout = [1 + (gm + gmb)Rs]rds.

Why does gm and gmb have anything to do with the output resistance when you are opening the current sources to find it?

Thanks