مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4209
Musnad of Abdullah bin Mas'ood
It was narrated from 'Abdullah that The Messenger of Allah (ﷺ) prayed five rak'ahs in Zuhr, and it was said to him: O Messenger of Allah, has something been added to the prayer? He said: “Why is that?” They said: You prayed five. And he turned around and prostrated twice after he had said the salam.
Musnad Ahmad 4237lts isnad is saheeh, al-Bukhari (1226) and Muslitn (572)]