Sahih al-Bukhari 4806Book 65, Hadith 328
حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنَا غُنْدَرٌ، حَدَّثَنَا شُعْبَةُ، عَنِ الْعَوَّامِ، قَالَ سَأَلْتُ مُجَاهِدًا عَنِ السَّجْدَةِ، فِي ص قَالَ سُئِلَ ابْنُ عَبَّاسٍ فَقَالَ {أُولَئِكَ الَّذِينَ هَدَى اللَّهُ فَبِهُدَاهُمُ اقْتَدِهْ}. وَكَانَ ابْنُ عَبَّاسٍ يَسْجُدُ فِيهَا.
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).