It is narrated from Ikrimah (may Allah have mercy on him) that Sayyiduna Ibn Abbas (may Allah be pleased with them both) was asked about the prostration in Surah Sad [Surah Sad: 1], and he said: Surah Sad is not among the surahs in which prostration is obligatory, but the Messenger of Allah (peace and blessings be upon him) used to prostrate in Surah Sad.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3739