Sad is not among the obligatory prostrations, but I saw the Messenger of Allah ﷺ prostrate during it.
حدثنا موسى بن إسمعيل حدثنا وهيب حدثنا أيوب عن عكرمة
ليس ص من عزائم السجود وقد رأيت رسول الله ﷺ يسجد فيها
Bukhari narrated it in Sujud al-Quran, chapter on the sajdah of Surah Sad, hadith no. 1069, from the hadith of Ayub, through him.