"a man entered the masjid, and the Messenger of Allah ﷺ was sitting in a corner of the mosque. He prayed, then he came and greeted him with Salam(peace), and he said: 'Wa 'alaikassalm."
أَنَّ رَجُلاً دَخَلَ الْمَسْجِدَ وَرَسُولُ اللَّهِ ﷺ جَالِسٌ فِي نَاحِيَةِ الْمَسْجِدِ فَصَلَّى ثُمَّ جَاءَ فَسَلَّمَ فَقَالَ وَعَلَيْكَ السَّلاَمُ
"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.
دَخَلَ رَجُلٌ الْمَسْجِدَ وَرَسُولُ اللَّهِ ﷺ جَالِسٌ فِي نَاحِيَةِ الْمَسْجِدِ فَصَلَّى ثُمَّ جَاءَ فَسَلَّمَ عَلَيْهِ فَقَالَ رَسُولُ اللَّهِ ﷺ وَعَلَيْكَ ارْجِعْ فَصَلِّ فَإِنَّكَ لَمْ تُصَلِّ فَذَكَرَ الْحَدِيثَ بِطُولِهِ