Shashwata

A dilemma has arisen regarding the first mantra related to the Samaveda Sandhya procedure. Why should the worshiping method start with this mantra from visnuh sukta- "Om Tad visnoh" (Rig Veda- 1/22/20)? Where did the procedure of sipping water by chanting "Om Visnuh" come from? Where can I find the reference of this procedure? Govila Grihyasutra clearly states that the achamana vidhi should start with "yasho asi" mantra. I am unable to find the reference books.