When the Imam is delivering the Khutba, a light two Rak'a Salat (prayer) (Tahayyat-ul Masjid)

Sahih al-Bukhari · Book 11 · Hadith 931

حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ اللَّهِ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ عَمْرٍو، سَمِعَ جَابِرًا، قَالَ دَخَلَ رَجُلٌ يَوْمَ الْجُمُعَةِ وَالنَّبِيُّ صلى الله عليه وسلم يَخْطُبُ فَقَالَ ‏"‏ أَصَلَّيْتَ ‏"‏‏.‏ قَالَ لاَ‏.‏ قَالَ ‏"‏ فَصَلِّ رَكْعَتَيْنِ ‏"‏‏.‏

Narrated Jabir: 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 rak`at."