Can I pray in my niqab? Is my prostration valid?

Two separate questions get tangled here. Take them apart.

Covering the face during prayer

Scholars are agreed that covering the face is not legislated in prayer in ordinary circumstances: some grade it disliked, others prohibited.

That changes when unrelated men can see her. There a number of jurists permit it, and some require it, following their position on covering the face from ajnabī generally.

Either way, the prayer itself is valid. Praying in niqab does not invalidate the prayer.

Prostration — the actual issue

The forehead and nose are the organs of prostration, and a niqab can come between the forehead and the ground. That is the basis on which some scholars prohibited praying in it altogether.

Imam al-Shāfiʿī's text distinguishes three cases. Prostrating on part of the forehead rather than all of it he disliked, but the prayer need not be repeated. Prostrating on the nose alone, without the forehead, does not count; the forehead is the place of prostration. Prostrating on the forehead with cloth beneath it does not count except where there is injury, which constitutes an ʿudhr — an excuse.

So a niqab covering the whole forehead invalidates the prostration; a niqab that leaves part of the forehead touching the ground, whether lifted, or simply not covering it, does not. The nose being covered is not a problem, since it need not touch.

The question actually being asked

Here is the part that a summary loses, and it is the interesting part.

Al-Shāfiʿī's exception is injury. The question is whether the presence of unrelated men is itself an ʿudhr of the same kind — whether a niqab worn to avoid being seen resembles a bandage on an injured forehead closely enough to inherit the concession.

The answer returns to the disagreement over covering the face from unrelated men in the first place. The prostration question does not resolve independently of it.

The silence of al-Khaṭīb al-Sharbīnī and others who forbade a woman praying near men from removing her niqab might suggest they treated it as a recognised excuse. Al-Bujayramī denies the inference: commenting on exactly that passage, he adds the condition that her forehead be exposed during prostration.

What to do

The recommended precaution is iḥtiyāṭ: uncover a small part of the forehead so it meets the ground directly.

That satisfies every position at once, which is why it is offered as practice rather than as a ruling. It requires no view on which school is correct, and no view on whether unrelated men constitute an excuse.