Praise be to Allaah.
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.
See also the answer to question no.
21803 and
1046.
And Allaah knows best.