Shaykh Saalih al-Fawzaan said:
When a woman is praying her whole body is ‘awrah and she must cover all of her body except her face, if there are no non-mahram men present.
If she is alone or with men who are her mahrams, then she may uncover her face when praying.
But if she is in the presence of non-mahram men, then she should cover her face when praying and at other times, because the face is ‘awrah.
Fataawah al-Mar’ah al-Muslim, 1/315.
And Allaah knows best.