I asked Muhajid regarding the prostration in Surat Saʿd. He said, "Ibn ʿAbbas was asked the same question and he said, 'Those are they (the prophets) whom Allah had Guided. So follow their guidance." (6.90) Ibn ʿAbbas used to perform a prostration (on reading this Sura).
{أُولَئِكَ الَّذِينَ هَدَى اللَّهُ فَبِهُدَاهُمُ اقْتَدِهْ} وَكَانَ ابْنُ عَبَّاسٍ يَسْجُدُ فِيهَا