Imam Al-Nawawi’s Riyad-us-Saliheen
Prohibition of Observing an Optional Saum (Fast) by a Woman without the
Permission of her Husband
1750. Abu Hurairah (May Allah be pleased with him) said: The Messenger of Allah (sallallaahu alayhi wa sallam) said, “It is not lawful for a woman to observe an optional Saum (fast) without the permission of her husband when he is at home. Nor should she allow anyone to enter his house without his permission.”
[Al-Bukhari and Muslim].