Narrated Abdullah ibn Abbas:

A woman made a voyage and vowed that she would fast one month if Allah made her reach her destination with peace and security. Allah made her reach her destination with security but she died before she could fast. Her daughter or sister (the narrator doubted) came to the Messenger of Allah (ﷺ). So he commanded to fast on her behalf.

حَدَّثَنَا عَمْرُو بْنُ عَوْنٍ، أَخْبَرَنَا هُشَيْمٌ، عَنْ أَبِي بِشْرٍ، عَنْ سَعِيدِ بْنِ جُبَيْرٍ، عَنِ ابْنِ عَبَّاسٍ، ‏:‏ أَنَّ امْرَأَةً، رَكِبَتِ الْبَحْرَ فَنَذَرَتْ إِنْ نَجَّاهَا اللَّهُ أَنْ تَصُومَ شَهْرًا، فَنَجَّاهَا اللَّهُ فَلَمْ تَصُمْ حَتَّى مَاتَتْ، فَجَاءَتِ ابْنَتُهَا أَوْ أُخْتُهَا إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فَأَمَرَهَا أَنْ تَصُومَ عَنْهَا ‏.‏