Is it permissible for a worshiper suffering from a disc (herniation) to continue bowing (rukūʿ) and prostrating (sujūd), despite an involuntary "Ah" sound emanating from him, or should he abandon bowing and prostrating, or is the ruling based on the prevalence or absence of groaning?

If a man is harmed by bowing (ruku') and prostration (sujud), to the extent that his devotion (khushu') is lost, then he is excused from performing them. He should pray standing, then gesture for bowing while sitting, and then gesture for prostration to the best of his ability. As for moaning due to pain, it does not invalidate the prayer even if two letters are uttered.