If the Imam prolongs the Salat (prayer) and somebody has an urgent work or need and so he leaves the congregation and offers Salat alone.
حَدَّثَنَا مُسْلِمٌ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ عَمْرٍو، عَنْ جَابِرِ بْنِ عَبْدِ اللَّهِ، أَنَّ مُعَاذَ بْنَ جَبَلٍ، كَانَ يُصَلِّي مَعَ النَّبِيِّ صلى الله عليه وسلم ثُمَّ يَرْجِعُ فَيَؤُمُّ قَوْمَهُ.
Narrated Mu`adh bin Jabal: I used to pray the `Isha prayer with the Prophet (ﷺ) and then go to lead my people in the prayer.