What is the obligation upon someone who vows to observe the last ten days of Ramadan?

Primary text

If a person vows to observe the last ten days of Ramadan, both the nights and the days are obligatory upon him. This follows the principle that when Arabic speakers use the term 'ten' (ashra) in this context, they intend the full period encompassing both nights and days, as demonstrated by common usage, such as saying 'We traveled for ten' to mean ten days and nights.