From Sahih Bukhari (Hadith 4806):
Narrated Al-Awwam:I asked Mujahid 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)
Reference: Bukhari 4806