If something comes upon the Imam after the iqamah has been said Sunan an-Nasa'i 791
Arabic hadith text
أَخْبَرَنَا زِيَادُ بْنُ أَيُّوبَ، قَالَ حَدَّثَنَا إِسْمَاعِيلُ، قَالَ حَدَّثَنَا عَبْدُ الْعَزِيزِ، عَنْ أَنَسٍ، قَالَ أُقِيمَتِ الصَّلاَةُ وَرَسُولُ اللَّهِ صلى الله عليه وسلم نَجِيٌّ لِرَجُلٍفَمَا قَامَ إِلَى الصَّلاَةِ حَتَّى نَامَ الْقَوْمُ .
Translation
It was narrated that Anas said
"The Iqamah for prayer was said, and the Messenger of Allah (ﷺ) was conversing privately with a man, and did not commence the prayer until the people slept."