Mua'dh bin Hisham bin Aby معاذ بن هشام بن أبي عبد الله · Chapters on Seeking Permission

"A man enter the Masjid and performed Salat while the Messenger of Allah (ﷺ) was sitting at the back of the Masjid. Then he came to greet him, so the Prophet (ﷺ) said: "Wa Alaikum (and upon you); go back and pray for indeed you have not prayed.'" And he mentioned the Hadith in its entirety.