Ali ibn Abi Talib · The Chapters on Expiation

"Whoever makes a vow and does not state it specifically, the expiation (for such a vow) is the expiation for breaking an oath. Whoever makes a vow and is not able to fulfill it, the expiation for that is the expiation for breaking an oath. Whoever makes a vow and is able to fulfill it, let him do so."

a woman came to the Messenger of Allah (ﷺ) and said: "My mother has died, and she had made a vow to fast, but she died before she could fulfill it. The Messenger of Allah (ﷺ) said: 'Let her guardian fast on her behalf ."