al-Laith bin Sa'd الليث بن سعد · The Book of Oaths and Vows

"The Messenger of Allah said: 'The best of you are my generation, then those who come after them, then those whom after them, then those who come after them.' -I do not know if he said two times after him or three. Then he mentioned some people who betray and cannot be trusted, who bear witness without being asked to do so, who make vows and do not fulfill them, and fatness will prevail among them."

"A woman traveled by sea and vowed to fast for a month, but she died before she could fast. Her sister came to the Prophet and told him about that, and he told her to fast on her behalf."