Sahih al-Bukhari, Book 19, Hadith 49 (In-Book) ⓘ
Narrated Jabir bin `Abdullah: While delivering a sermon, Allah's Messenger said, "If anyone of you comes while the Imam is delivering the sermon or has come out for it, he should offer a two rak`at prayer."
حَدَّثَنَا آدَمُ، قَالَ أَخْبَرَنَا شُعْبَةُ، أَخْبَرَنَا عَمْرُو بْنُ دِينَارٍ، قَالَ سَمِعْتُ جَابِرَ بْنَ عَبْدِ اللَّهِ ـ رضى الله عنهما ـ قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم وَهُوَ يَخْطُبُ " إِذَا جَاءَ أَحَدُكُمْ وَالإِمَامُ يَخْطُبُ ـ أَوْ قَدْ خَرَجَ ـ فَلْيُصَلِّ رَكْعَتَيْنِ " .
also referenced under: Hadith 1170 (DarusSalam) & Volume 2, Book 21, Hadith 267 (USC-MSA)