Musnad Ahmad 1860

It was narrated from Ibn "Abbas that a woman travelled by sea and vowed that if Allah, may He be blessed and exalted, saved her, she would fast for a month. Allah, may He be glorified and exalted, saved her but she did not fast until she died. A relative of hers came to the Prophet (ﷺ) and told him about that, and he said:“ Fast(on her behalf).“