Regarding the prostration in Surah Sad, it is not among the obligatory prostrations, but I saw the Messenger of Allah ﷺ prostrate in it.
حدثنا إسماعيل حدثنا أيوب عن عكرمة
في السجود في ص ليست من عزائم السجود وقد رأيت رسول الله ﷺ يسجد فيها
Its isnad is sahih, it is a longer version of 2521, and Ibn Kathir quoted it in the Tafsir 7:193 from this passage, attributing it to Bukhari, Abu Dawud, Tirmidhi, and Nasai in his tafsir.