Al-Awwam (may Allah have mercy on him) narrated that Mujahid (may Allah have mercy on him) was asked about the prostration in Surah Sad, and he said: Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) was asked about it, and he said: "These are the guided people; follow their guidance" [Surah al-An'am: 90]. And Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) used to prostrate in it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3747