Musannaf Ibn Abi Shayba 7:389

Jarir narrated to us, from Mughira, from Ibrahim, regarding a woman who obligated upon herself and vowed to pray in fifty mosques, provided that she gives charity from fifty houses and gives it as charity. He ordered her not to give charity, for it is an act of disobedience; she should offer expiation for her oath and pray in the fifty mosques, because prayer is from the obedience of Allah.