ANSWER:

I think you mean this dua 👇 and this is authentic to recite as a dua in the last tashahhud, before making salam.

It was also narrated that the Prophet (peace and blessings of Allah be upon him) taught Abu Bakr al-Siddiq a du’a which he could recite during his Salah, as was narrated by al-Bukhari. Abu Bakr al-Siddiq (may Allah be pleased with him) said to the Messenger of Allah (peace and blessings of Allah be upon him), “Teach me a du’a that I can recite during my salah.” He said: “Say: ‘Allahumma inni zalamtu nafsi zulman kathiran wa la yaghfir ul-dhunoob illa anta, faghfir li maghfiratan min ‘indaka warhamni, innaka anta al-Ghafoor al-Rahim (O Allah, indeed I have wronged myself greatly, and there is none who forgives sins besides You. So grant me forgiveness from You and have mercy on me, for You are the All-Forgiving, Most Merciful).’” (790)

QUESTION: I was taught to recite this during the last rakahs of prayers before ending the prayer and after “Allahumah sali Allah Muhammad…” it goes “Allahumah inni Salam tul nafsir…” I don’t know if you know this though but when I grow up I realise ppl just end the prayer without saying this. Please advice me on this matter.