Thank you very much for your question. It is our pleasure to answer it.

`A'ishah (may Allah be pleased with her) narrated that the Prophet (peace be upon him) said, "Allah does not accept the prayer of an adult woman unless she is wearing a head-covering (khimar, jilbaab, hijab, etc.)." (Agreed Upon) 

For the prayer of a believing woman to be acceptable, she must cover herself completely in her khimar or hijaab or jilbab, just as she would when in the presence of non-mehram members of the opposite sex, even if she were to pray absolutely alone.

It was narrted that Asma', daughter of Abu Bakr, entered upon the Messenger of Allah (peace be upon him) wearing thin clothes. The Messenger of Allah (peace be upon him) turned his attention from her and said: O Asma', when a woman reaches the age of menstruation, it does not suit her that she displays (to non-mehram men) her parts of body except this and this, and he (saws) pointed to her face and hands." (Agreed upon) 

Because the hands from the wrists onwards do not form part of the `awrah of the believing woman, she is not obliged to cover them when in the presence of non-mehram males or during her prayers. But if for any reason the woman wishes to cover her hands while offering her prayers, there is absolutely no harm and it would not effect the validity of her prayers. And Allah Alone Knows Best.

I hope this answers your question.