The prostration of Sūrah Sād is not a compulsory one but I saw the Prophet (may Allah's peace and blessings be upon him) prostrating while reciting it

The meaning of the Hadīth: "The prostration of Sād is not a compulsory one". It means that the prostration of reciting the Sūrah Sād is Sunnah and not obligatory, because no emphatic instruction has been reported from the Prophet concerning it. Rather, the wording narrated is in the form of a report, that Dawūd (peace be upon him) prostrated it as a repentance to Allah.

And our Prophet (may Allah's peace and blessings be upon him) prostrated out of thankfulness; because Allah blessed Dawūd (peace be upon him) with forgiveness. There is another supporting evidence in Sunan An-Nasā'i in its favor, which is reported as the Prophet (may Allah's peace and blessings be upon him) said: "Dawūd prostrated in this verse, out of repentance, and we prostrate it out of thankfulness to Allah."