Narrated Al-Awwam:
I asked Muhajid regarding the prostration in Surat Sa`d. He said, "Ibn `Abbas was asked the same question and he said, 'Those are they (the prophets) whom Allah had Guided. So follow their guidance." (6.90) Ibn `Abbas used to perform a prostration (on reading this Sura).
USC-MSA web (English) reference: Vol. 6, Book 60, Hadith 330
Arabic reference: Book 65, Hadith 4806