مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3547
Musnad of Abdullah bin Mas'ood
It was narrated from ‘Abdullah (رضي الله عنه) that The Prophet (ﷺ) did the two prostrations after the salam. And on one occasion he said: The Prophet (ﷺ) did the two prostrations of forgetfulness after the salam.
Musnad Ahmad 3570Its isnad is saheeh, al-Bukhari (1226) and Muslim (572)]