This article is from the California Driving (and Surviving) FAQ, by "George J Wu" firstname.lastname@example.org with numerous contributions by others.
There's no CVC section requiring it (see FAQ on lane sharing). On the other
hand, some people (including police officers, acting in their official
capacities) interpret the requirement to keep as far to the right "as
practicable" of CVC 21202(a) to require bicycles to ride single file. Under
this interpretation, unless passing, turning, avoiding road debris, etc.
(see quote of CVC 21208 above), the cyclist on the left is not as far right