found the link at last

As with undertakings from an applicant (see above), the ECO should only require an undertaking from a sponsor in exceptional circumstances or for certain categories eg parents, grandparents and other dependent relatives under Rule 317 of the Immigration Rules. (see Maintenance & Accommodation - MAA)
ECB12.6 Is failure to provide / comply with an undertaking on maintenance and accommodation grounds for refusal?

Yes. Failure by a sponsor to provide an undertaking on maintenance and accommodation when requested is grounds for refusal (Rules paragraph 320(14))
https://www.gov.uk/government/public...2-undertakings