Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said that a woman, while she was at sea, vowed that if Allah saved her she would fast for a month. Allah saved her, but she died before she could fast. One of her female relatives (her sister or her daughter) came to the Messenger of Allah (peace and blessings be upon him) and informed him, so he (peace and blessings be upon him) said: "Fast on her behalf."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8229