If someone is certain or deems it probable that water is harmful for him and performs tayammum, and before prayers he realises that water is not harmful for him, his tayammum is void. If he realises this after prayers, he must perform the prayer again with wuḍūʾ or ghusl unless performing wuḍūʾ or ghusl while being certain of harm, or deeming it probable, would cause anxiety which would be difficult to endure.