There are many dua’s which can be recited during prayer according to the Sunnah of Prophet Muhammad (ﷺ).
Abu Bakr As-Siddiq (May Allah be pleased with him) reported:
I requested the Messenger of Allah (ﷺ) to teach me a supplication which I could recite in my Salat (prayer). Thereupon he said, “Recite: Allahumma inni zalamtu nafsi zulman kathiran, wa la yaghfirudh- dhunuba illa Anta, faghfir li maghfiratan min ‘indika, warhamni, innaka Antal-Ghafur-ur-Rahim
Riyad As Salihin Arabic/English Book 17, Hadith 1475.
[Al-Bukhari and Muslim]
Full Dua in Arabic Text:
اللهم إني ظلمت نفسي ظلمًا كثيرًا، ولا يغفر الذنوب إلا أنت، فاغفر لي مغفرة من عندك، وارحمني، إنك أنت الغفور الرحيم
Translation in English
O Allah! I have considerably wronged myself. There is none to forgive the sins but You. So grant me pardon and have mercy on me. You are the Most Forgiving, the Most Compassionate.
These duas for salat are not mandatory but rather are recommended. There are many duas (far too many to cover in a single post) which can be recited during salat according to the sunnah of Prophet Muhammad (ﷺ).
The virtues or benefit of this specific dua:
“Your god is one God. But those who do not believe in the Hereafter – their hearts are disapproving, and they are arrogant.” (Surah Nahl Ayat 22)
Being prideful, boastful, or showing arrogance, meaning to exaggerate one’s sense of importance or abilities, is one of the biggest sins or character flaws mentioned in the Quran repeatedly. Allah loves the one’s who are humble and show humility. Knowing the meaning of this dua you admit to your faults, you begin the dua by saying “O Allah, I have wronged myself” and continues by saying only Allah will be able to pardon my shortcomings.
AL-GHAFOOR is name meaning the all-forgiving.
AR RAHEEM is the name meaning the merciful. The one who acts with extreme kindness.
Same Dua From Sahih Al Bukhari Hadith #834
Narrated Abu Bakr As-Siddiq:
I asked Allah’s Messenger (ﷺ) to teach me an invocation so that I may invoke Allah with it in my prayer. He told me to say,
Allahumma inni zalumtu nafsi zulman kathiran, Wala yaghfiru dh-dhunuba illa anta, fa ghfir li maghfiratan min `indika, wa r-hamni, innaka anta l-ghafuru r-rahim
O Allah! I have done great injustice to myself and none except You forgives sins, so bestow on me a forgiveness from You, and Have Mercy on me, You are the Forgiver, the Merciful.