The Prophet ﷺ said: When an imam stands up at the end of two rak'ahs , if he remembers before standing straight up, he should sit down, but if he stands straight up, he must not sit down, but perform the two prostrations of forgetfulness. Abu Dawud said: I have not narrated in this book of mine any hadith from Jabir Al-Ju'fi (one of the narrators) except this one. (Using translation from Abū Dāʾūd 1036)   

قَالَ رَسُولُ اللهِ ﷺ إِذَا قَامَ الْإِمَامُ فِي الرَّكْعَتَيْنِ فَإِنْ ذَكَرَ قَبْلَ أَنْ يَسْتَتِمَّ قَائِمًا فَلْيَجْلِسْ وَإِنِ اسْتَتَمَّ قَائِمًا فَلَا يَجْلِسْ وَيَسْجُدْ سَجْدَتَيِ السَّهْوِ  

[Machine] He moved to perform the two units of prayer for Asr, so they glorified him, then he sat and performed the two prostrations of forgetfulness while he was sitting.  

أَنَّهُ تَحَرَّكَ لِلْقِيَامِ فِي الرَّكْعَتَيْنِ مِنَ الْعَصْرِ فَسَبَّحُوا بِهِ فَجَلَسَ ثُمَّ سَجَدَ سَجْدَتَيِ السَّهْوِ وَهُوَ جَالِسٌ