Say Hundred times:
اَللَّهُمَّ اغْفِرْ لِي وَتُبْ عَلَيَّ إِنَّكَ أَنْتَ التَّوَّابُ الرَّحِيْم (التَّوَّابُ الْغَفُورُ)
Pronunciation
Allahum-maghfir lee wa tub `alayya innaka Antat-Tawwabur-Rahim (in another narration: Tawwabul-Ghafur)
Meaning
O Allah, forgive me and turn to me. You are the Ever-Turning, the Merciful' (in another narration: the Most Forgiving) "An Ansari Companion (RA) said: "I have heard the Messenger of Allah (ﷺ) say this dua 100 times after the prayer." The second narration of this hadith says: “Rasulullah (ﷺ) prays two rak'ats of Salatud-Duha. Then he recited this dua 100 times. Both descriptions are authentic. At least all the Dhaakirs should pay attention to recite this Dua 100 times after Salatud Duha.
Source: Sahih (Albani). Sahihah: 556, Ahmad: 5564
Duas from IslamicAPI's Masnun Dua dataset (based on the Quran, hadith, Hisnul Muslim and Rahe Belayat). See the dataset