مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3363
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated from Ibn ‘Abbas (رضي الله عنه) that He said concerning the prostration in (Soorat) Sad: It is not among the compulsory prostrations, but I saw the Messenger of Allah (ﷺ) prostrating in it.
Musnad Ahmad 3387Its isnad is saheeh, al-Bukhari (1069)]