What is the ruling on the prayer of one who doubted whether the prostration he performed was the first or the second, so he immediately performed a second prostration without any mention between the two prostrations, then completed his prayer and performed the prostrations of forgetfulness, doubting his first prostration from it, and said: "Peace be upon you and the mercy of Allah" while prostrating?

Avoiding and disregarding whisperings (waswas) is the most beneficial cure for them. The ذكر (remembrance/supplication) between the two prostrations is a sunnah according to the majority of scholars. If one omits it, whether intentionally or forgetfully, their prayer remains valid. Forgetfulness during the prostration of forgetfulness (sujud al-sahw) should be disregarded and does not invalidate the prayer. If one's doubts become frequent to the point of resembling whisperings, they should be ignored, and no prostration of forgetfulness is required for them.