Hello bro...!!!
I haven't found such documentation on cic, I have also relied much on what people says on this forum, and in my case that wasn't required as well. However since you asked me the question i tried to search on cic, but i didn't find anything written in black and white. But i came across manual stating the eligibility requirement for accepting the application. I am attaching the link for the same
Please go to point number 12.1 last para page number 24 ...where they have mentioned.
"Pursuant to R77, the requirements and criteria set forth in R76 must be met at the time the
application is made, as well as when the permanent resident visa is issued. If the applicant is
unable to demonstrate that they have sufficient funds available to meet the requirements, the
officer should refuse the application and proceed to section 14.2."
Here i am interpreting that they have mentioned "applicant" have to demonstrate the POF and as per the form, your spouse is also an applicant. Further they have mentioned "they have sufficient fund", interpreting that if it was intended for prinicipal applicant then why should they write "they".
Forgive me if the answer is not upto your expectation but this is what i could provide best. I will keep searching, if get i will let you know.
Thank you