Is bob hairstyle permissible for women? In this type of hairstyle the hairs will be left untied and will be just above shoulders and below ears. Is the prayer of the woman who prays with untied hair valid?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Covering the 'Awrah is among the necessary conditions for the validity of the prayer. In prayer, the whole body of the woman is from the ‘Awrah except her face and hands. This is the prevalent opinion of Muslim scholars. Therefore, a woman has to cover her hair in the prayer. The Prophet said: “Allah does not accept the prayer of a pubescent woman unless she is wearing a head covering (Khimar).” If a woman covers her whole hair so that no hair comes to view, then her prayer is valid; tying the hair is not necessary. However, if her whole hair or some of it gets exposed then she should repeat her prayer.

Allah knows best.