Sayyiduna Ibn Abbas (may Allah be pleased with them both) said regarding the prostration in Surah Sad: This is not among the obligatory prostrations of recitation, and I have seen the Messenger of Allah (peace and blessings be upon him) prostrate upon it.
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1506
(Commentary on Ahadith 1504 to 1506) In Surah Sad, the verse of prostration is: «﴿فَاسْتَغْفَرَرَبَّهُوَخَرَّرَاكِعًاوَأَنَابَ﴾ [ص : 24]». That is, the state of Dawud (alayhis salam) falling into prostration has been described; therefore, Imam Shafi'i rahimahullah did not include this among the verses of prostration of recitation (sujud at-tilawah), as is also narrated from Sayyiduna Abdullah ibn Abbas radi Allahu anhuma in the following narration.