I asked Mujahid about the prostration in Surah Sad. He said that Ibn Abbas was asked about it and replied, "Those are the ones whom Allah has guided, so follow their guidance" [Anam 6:90]. Ibn Abbas used to prostrate during it.
حدثنا محمد بن بشار حدثنا غندر حدثنا شعبة
سألت مجاهدا عن السجدة في ص قال سئل ابن عباس فقال {أولئك الذين هدى الله فبهداهم اقتده} وكان ابن عباس يسجد فيها