Well actually it depends on whether you derive the formula for Vni(Input referred noise with or without Rs(source resistance). Ultimately Rs affects Vni. There is a minimum Rs that gives minimum noise but since Rs is fixed you have no choice. Some books take Rs into consideartion while some don't when deriving Vni.
Refer the following book for any noise related questions. I found it very helpful when designing a LNA.
LOW NOISE ELECTRONIC SYSTEM DESIGN
Author: MOTCHENBACHER
ISBN: 04715-7742-1