Q. Is there any documentation that can support this?
Here's a link to New York's memorandum on its "convenience of the employer" tax doctrine regarding non-resident telecommuters: https://www.tax.ny.gov/pdf/memos/income/m06_5i.pdf
Also, see these previous discussions (which include other references), wherein posters say it requires at least 1 day in NY to be subject to NYS income tax, even when it's for your own convenience. https://ttlc.intuit.com/community/state-taxes/discussion/re-starting-new-job-with-office-in-new-york...
https://ttlc.intuit.com/community/taxes/discussion/re-will-i-owe-any-california-income-tax/01/276936...
https://ttlc.intuit.com/community/state-taxes/discussion/re-i-work-for-a-ny-company-remotely-from-nc...
https://ttlc.intuit.com/community/state-taxes/discussion/new-york-state-taxes-for-md-remote-employee...
