Jami` at-Tirmidhi 2692
جامع الترمذي#2692SahihNarrated Abu Hurairah:"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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Ahmad Muhammad ShakirSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih - Bukhari And Muslim
In-book reference: 42:5