Sahih al-Bukhari 1170

Arabic

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 ‏"‏‏.‏

English translation

Narrated Jabir bin `Abdullah:While delivering a sermon, Allah's Messenger (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 rak`at prayer

Reference: Sahih al-Bukhari 1170, Book of Prayer at Night (Tahajjud). English translation by Dr. Muhammad Muhsin Khan.