(32) باب إِذَا رَأَى الإِمَامُ رَجُلاً جَاءَ وَهْوَ يَخْطُبُ أَمَرَهُ أَنْ يُصَلِّيَ رَكْعَتَيْنِ
Narrated Jabir bin `Abdullah:
A person entered the mosque while the Prophet (ﷺ) was delivering the Khutba on a Friday. The Prophet (ﷺ) said to him, "Have you prayed?" The man replied in the negative. The Prophet (ﷺ) said, "Get up and pray two rak`at."
Reference: Sahih al-Bukhari 930
In-book reference: Book 11, Hadith 54
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 13, Hadith 52