Mujahid (may Allah have mercy on him) narrated that Ibn Abbas (may Allah be pleased with them both) was asked about the prostration in Surah Sad, and he said: "These are the people whom Allah guided, so follow their guidance" [Surah al-An'am: 90].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3746