Sahih al-Bukhari 1170
Sahih al-Bukhari · Book 19: Prayer at Night (Tahajjud) · What is recited in the two Rak'a (Sunna) of the Fajr
Narrated Jabir bin `Abdullah:
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 " .
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."