Say Hundred times:
اَللَّهُمَّ اغْفِرْ لِي وَتُبْ عَلَيَّ إِنَّكَ أَنْتَ التَّوَّابُ الرَّحِيْم (التَّوَّابُ الْغَفُورُ)
Transliteration
Allahum-maghfir lee wa tub `alayya innaka Antat-Tawwabur-Rahim (in another narration: Tawwabul-Ghafur)
Translation
O Allah, forgive me and turn to me. You are the Ever-Turning, the Merciful' (in another narration: the Most Forgiving)
Note
An Ansari Companion (RA) heard the Messenger of Allah ﷺ say this dua 100 times after prayer. Another narration says: Rasulullah ﷺ prays two rak'ats of Salatud-Duha, then recites this dua 100 times. Both are authentic. Those who do Zikr should try to say this Dua 100 times after Salatud Duha.
After salat, this dua brings Allah's forgiveness and mercy. Every muslim can find hope and a clean heart after each prayer by trusting Allah's kindness and His power to accept tawbah.
After muslim prayer, Prophet Muhammad said: Nothing deserves worship but Allah Alone. He has no partner. The kingdom belongs to Him. All praise goes to Him. He has power over everything.
When salah ends, say this after Tashahhud: Allahumma inni a'udhu bika min 'adhabi. The Prophet taught it - asking Allah for protection from four things.
A masnoon dua works after every namaz. Like this: In the Name of Allah, peace be on Mohammed and his family, and I give my work to Allah, who sees all things.
Allahumma inni a'udhu bika min zawali ni'matika pleases Allah. You ask Him to keep His blessings, safety, and not lose what makes Allah happy with you.
After fard prayer, the Prophet said: Nothing deserves worship but Allah Alone. No partner for Him. His is the kingdom. His is all praise. He controls everything.