A woman traveled by sea and vowed that she would fast for a month, but she died before she could fast. Her sister came to the Prophet ﷺ and mentioned this to him, so he instructed her to fast on her behalf.
أخبرنا بشر بن خالد العسكري قال حدثنا محمد بن جعفر عن شعبة قال سمعت سليمان يحدث عن مسلم البطين عن سعيد بن جبير
ركبت امرأة البحر فنذرت أن تصوم شهرا فماتت قبل أن تصوم فأتت أختها النبي ﷺ وذكرت ذلك له فأمرها أن تصوم عنها
[Its isnad is sahih] Ahmad narrated it, 1/338, from Muhammad bin Jafar Ghundar through the same chain, and it is in Al-Kubra, h: 4758, and Ibn Khuzayma graded it sahih, h: 2054, and Abu Dawud narrated it, h: 3308, from the hadith of Saeed bin Jubayr through the same chain.