Do you want to know what dua the Prophet (PBUH) made after salah? Whenever the Messenger of Allah (ﷺ) concluded his prayer, he would beg forgiveness from Allah 3 Times.

استغفراللہ

“Astaghfirullah”

I seek forgiveness of Allah.

And then would recite:
اللهم أنت السلام ومنك السلام تباركت يا ذا الجلال والإكرام

“Allahumma Antas- Salamu, wa minkas-salam, tabarakta ya zal-Jalali wal-Ikram.

O Allah, You are the Grantor of security, and security comes from You. You are Blessing, O You Who have majesty and nobility.

[Sahi Muslim Book 16, Hadith 1415]..