Jami` at-Tirmidhi 789
جامع الترمذي#789Da'ifAishah narrated that:The Messenger of Allah said: "Whoever stays with a people, then he is not to fast without their permission
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Ahmad Muhammad ShakirVery Daif
Al-AlbaniVery Daif
Bashar Awad MaaroufDaif
Zubair Ali ZaiDaif
In-book reference: 8:108