While you are still sitting where you prayed
The angels keep asking for you as long as you stay in the place you prayed.
الْمَلاَئِكَةُ تُصَلِّي عَلَى أَحَدِكُمْ مَا دَامَ فِي مُصَلاَّهُ الَّذِي صَلَّى فِيهِ، مَا لَمْ يُحْدِثْ، تَقُولُ اللَّهُمَّ اغْفِرْ لَهُ اللَّهُمَّ ارْحَمْهُ
Allāhumma-ghfir lah, Allāhumma-rḥamh.
“O Allah, forgive him. O Allah, have mercy on him.”
This is not a du'ā for you to say. It is what is being said about you, by others, while you remain sitting after the prayer.
The full narration
Narrated Abu Huraira:Allah's Messenger (ﷺ) said, "The angels keep on asking Allah's forgiveness for anyone of you, as long as he is at his Musalla (praying place) and he does not pass wind (Hadath). They say, 'O Allah! Forgive him, O Allah! be Merciful to him
Source
Ṣaḥīḥ al-Bukhārī · 445
Ṣaḥīḥ
- Agreed upon as authentic (al-Bukhārī / Muslim)