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).
Sahih al-Bukhari Book 10, Hadith 227
← Previous Hadith (the wife of the Prophet) Allah's Messenger (ﷺ) used to invoke Allah in the prayer saying "Allahumma inni a`udhu bika min `adhabi l-qabr, wa a`udhu bika min fitnati l-masihi d-dajjal, wa a`udhu bika min fitnati l-mahya wa fitnati l-mamat. Allahumma inni a`udhu bika mina l-ma'thami wa l-maghram. (O Allah, I seek refuge with You from the punishment of the grave, from the afflictions of the imposter- Messiah, and from the afflictions of life and death. O Allah, I seek refuge with You from sins and from debt)." Somebody said to him, "Why do you so frequently seek refuge with Allah from being in debt?" The Prophet (ﷺ) replied, "A person in debt tells lies whenever he speaks, and breaks promises whenever he makes (them)." `Aisha also narrated: I heard Allah's Messenger (ﷺ) in his prayer seeking refuge with Allah from the afflictions of Ad-Dajjal.

Sahih al-Bukhari Book 10, Hadith 226 Next Hadith → When we prayed with the Prophet (ﷺ) we used to say, "Peace be on Allah from His slaves and peace be on so and so." The Prophet (ﷺ) said, "Don't say As-Salam be on Allah, for He Himself is As-Salam, but say, at-tahiyatu li l-lahi wa s-salawatu wa t-taiyibat. As-salamu `alaika aiyuha n-Nabiyu wa rahmatu l-lahi wa barakatuh. As-salamu `alaina wa `ala `ibadi l-lahi s-salihin. (If you say this then it will reach all the slaves in heaven or between heaven and earth). Ash-hadu al la-ilaha illa l-lah, wa ash-hadu anna Muhammadan `Abduhu wa Rasuluh.' Then select the invocation you like best and recite it." (See Hadith No. 794, 795 & 796).

Sahih al-Bukhari Book 10, Hadith 228