A man entered the Mosque while the Prophet ﷺ was delivering the Khutba. The Prophet ﷺ said to him, "Have you prayed?" The man replied in the negative. The Prophet ﷺ said, "Pray two rakʿat."  

دَخَلَ رَجُلٌ يَوْمَ الْجُمُعَةِ وَالنَّبِيُّ ﷺ يَخْطُبُ فَقَالَ: «أَصَلَّيْتَ؟» قَالَ: لاَ۔ قَالَ: «فَصَلِّ رَكْعَتَيْنِ۔»  

While delivering a sermon, Messenger of Allah ﷺ said, "If anyone of you comes while the Imam is delivering the sermon or has come out for it, he should offer a two rakʿat prayer."  

قَالَ رَسُولُ اللَّهِ ﷺ وَهُوَ يَخْطُبُ إِذَا جَاءَ أَحَدُكُمْ وَالإِمَامُ يَخْطُبُ أَوْ قَدْ خَرَجَ فَلْيُصَلِّ رَكْعَتَيْنِ